Being Prot has been fantastic. It's like being the Belle of the Ball. Everybody wants you. :D
No, I'm still very much an Arms Warrior. I'm always gonna be an Arms Warrior. But I really like switching roles a lot better now... way better than in TBC. I'm gonna share the experience from an Arms Warrior point of view. There are plenty of starter tank guides out there, and MUCH better Warrior tanks to learn from, so the focus here is on the Arms Warrior who specs Prot from time-to-time, gets called to tank in their Arms spec occasionally, and any Warrior who hasn't spent a lot of time in the tank roll. As usual, I'm also writing from a more "casual" player's view.
Which means I'm gonna ramble a lot, so hang on... :P
The Go To Guy/Girl
So, I'm in a UK PuG and the group leader is a Rogue. The whole party consists of me (Prot Warrior), a Death Knight, two Rogues, and a Holy Priest. All melee DPS. The lead Rogue just starts marking targets and sap'ing right from the start. The Priest is giving directions since she's the only one that's run it before. Not much for me to do except gather mobs and hold 'em still while the DPS chops 'em up. Easy enough. By the end of the run, though, I'm calling the shots, deciding how to approach the bosses, tanking without cc (yes, those doggies can see you my little Rogue friends) and so on. I didn't ask to be in charge. It just kinda happens when you're the tank.
So the first thing an Arms Warrior has to get used to when tanking is being the leader. Not just the center of focus. Grabbing and holding aggro is LOT different than being focused on a battlefield. Getting attacked from all sides and killing everything around you is not the same as controlling the situation. That's exactly what a tank does. The tank has to do everything they can to control what's going on around them. And when it goes to wtf hell (and it will), you have to get control back quickly. It's an entirely different set of skills. As an Arms Warrior you survive and kill and thrive in unpredictability and chaos. As a tank you control.
What will help you tremendously in this situation is your awareness. Arms Warriors have a lot of practice keeping track of enemies (mobs or other players) and tuning into what party members are doing. They have to if they want to stay alive. While the technical skill set you use when tanking is considerably different, your ability to stay on top of what's going on around you should already be built in. It's probably the single most important skill a tank can have, so if you've been Arms for awhile, you're way ahead.
This first thing you have to be aware of is threat. You DPS. Well, now you TPS. That's Threat Per Second. You need to ooze threat.
This is actually an easy switch if you've ever DPS'd a boss fight. Where before you had to work constantly to NOT pull aggro, now you can cut loose. So, forget what you know. You don't need to ramp up your damage slowly in a dungeon. You don't need to hold back your cool downs. Unload right from the start. Now you can't have enough threat.
What's nice, from the Arms perspective, is that you should already know the difference between holding aggro and pulling aggro. (And if you don't, shame on you... LEARNIT DERNIT!) As DPS you have to watch your threat so you don't pull aggro off your tank. As the tank, you WANT to pull aggro off another player. In fact, if you don't, people will be very, very, very upset with you. Yes, even if it's their fault. But since you're Arms, you should be able to know when it is your fault and when it's not. (ie. ranged dps targeting the wrong mob in the kill order vs. you missing that Devastate on the third mob from the left.) Knowing this will make you a better tank.
As pointed out, if you're gonna tank, people are gonna expect you to lead. So I made a checklist. It may seem obvious, but now you're not doing this just for yourself, you've gotta make sure the whole party is on board and on the same page. (Stayed tune, more mixed metaphore recipes coming soon.)
For example, while in Nexus, the PuG I was running with was doing great. Mowing down trash easily, and nailing the bosses just fine. Then we got a four pull. And I'm thinking no problemo, I mark and get to work. Next thing I know our Arms Warrior is dead and clearly a little upset with me. She was hitting the third mob in the kill order, and the cross was still up. I do skull, then cross, mark the cc, mark the pat and don't mark anything past that for simplicity in a 5-man. It's skull, cross and then kill left to right. And she was right to be upset. No, she wasn't hitting the wrong mob. I realized that I never said a word about kill order. Not once. It was going so well, I made the horrible mistake of just assuming. That mistake added another few gold to her repair bill.
Tank's Group Checklist
1. Make sure everyone is fully repaired and ready to roll BEFORE they are summoned. There's nothing worse than a player who runs out mid-instance to repair. Yah, after five wipes and the dwarf is down to using his blacksmith hammer, go ahead and send people out to repair. After a beating like that, everyone needs to take ten anyhoo. But there's no excuse for someone coming in without pristine stuff on their bod.
2. Establish your marking system right away or someone's gonna die. Nuff said.
3. Establish loot rules. Keep it simple. Need before Greed works fine for PuG and casual 5-man/10-mans. Don't make it more complex than necessary. Short version to macro: "I suggest loot as Need before Greed. Need only on main spec upgrades and please ask first. Greed on everything else. Unclaimed BoP to enchanter for disenchant /roll. Agreed?"
(Don't be a nazi about this, btw. If someone asks to Need an item that is off spec, let 'em if no one needs it for their main spec. For BoP it's generally Main Spec, then Off Spec, then Disenchant.)
4. Related to the above. Ask if anyone is going in for something specific and establish if a roll is gonna be needed before hand. Save yourself the headache of having to do it after the fight. Because it'll lead right into another fight... one that doesn't involve a boss.
5. Bring basic supplies. This is actually optional and it's a good idea, when you make sure everyone is repaired, to make sure they have the basics, as well. I bring an extra stack of basic vendor food and, being an alchemist, the mats to make mana and healing pots as needed. Don't get crazy with this. Bumming a couple of mana pots on the final boss is fine. Expecting someone to bring them for you for the entire run is a bit much. Everyone should have heal/mana pots, basic class elixirs/flasks, buff food and bandages. They should also bring any reagents they need for special spells. Check before you start!
6. Have everything you need. Yes, you actually have to set a good example. :)
7. If you don't know the dungeon well, ask for strategy help from anyone who's been in before. It's a good idea to read up and watch any videos you can find about the dungeon you're gonna run. The more you know...
8. Say thank you. Thank the entire party for the group at the end. Send a whisper to the healer thanking them specifically and personally. Even in a bad run, they'll remember it and probably heal for you again. :)
You fight differently as a Prot Warrior, and this is where the technical differences come in. You don't ever fight alone.
You have to be aware of other classes abilities so that you can call on them when needed. Arms Warriors tend to be most concerned with class abilities that can either keep them alive longer, or hold a bad guy still long enough for us to kill 'em. As tank, I think you really gotta know a bit more about each class' abilities. In one run, for example, I completely forgot that Shammies can cc. Oops. Finally he spoke up and I started calling for it when needed. My bad. If you don't know, ask. You don't have to sound like a noob. "Can anyone cc that caster on the left?" is fine.
Tank Tip: Even the most difficult player wants to contribute. If you have a player that's being a pain, give 'em a job. You'd be surprised how far including someone will go.
One of the biggest mistakes I see Arms Warriors make constantly in PvE is Charging at the start of a fight and pounding away on the mob. (Is it just me or is Charge now automatically placing us behind the mob???) We're very use to beating the snot out of anything in front of us, head on. If you're in font of the mob, it's very bad. You're just killing your tank cause the mob gets a free shot (kinda like an Overpower) on your tank every time it Parries one your hits. Well... if you tank, you don't have to worry about getting behind the mob. :)
You do, however, have to worry about everyone else getting behind the mob. Hunter pets and Rogues are very good at this. Pets automagically go into the mob's back if they don't have aggro. Rogues are just use to doing it automatically because they get a lot of practice at it. Instruct other melee DPSers to stand on the pet, or stand on the Rogue. It works.
You need to study up on how threat is generated. DPS is important, but not nearly as much as Threat. So, if Shield Slam (big threat) is on cool down and Revenge isn't up, you throw Devestate (the enhanced Prot version of Sunder Armor). It doesn't do as much damage, but it's big time threat. Sometimes bigger damage is better, though. When Sword & Board pops, smash 'em in the face. Twice.
You're still gonna use your DPS meter, but now you're gonna switch it to TPS. It'll give you a good overview of how much threat you're putting out. More important, it'll give you a quick look at who is putting out the most. I use Recount, but any DPS meter that will do TPS will do just fine. While spec'd Prot, throw Vigiliance on that party memebr. It's tempting to always throw it on the healer, which may indeed be the best place for it. But if the healer isn't generating much threat during the fights, it might be better to put it on someone else. Just lately, I've found Death Knights in particular tend to pull threat quickly. Those frosty ones are terrible about it.
You do, however, have to keep your threat meter up. The DPS meter won't substitute for a good threat meter because ya gotta know where that threat is coming from and where aggro is going before it actually goes. Only a good threat meter will tell you that. The built in one Blizz added with 3.0 is okay, but an add-on like Omen is much better and can give you a quick look at threat on each target. And even warn you when you're about to loose aggro.
Blizz gave all Warriors a little treat when they made Shield Slam a learned skill, rather than a Talent in the Prot tree. I'll go out on a limb and say that, combined with increased threat and damage from Thunderclap, has made Arms Warriors almost as good as TCB Prot Tanks! Well, maybe not that good. But definitely better. :)
When you tank as Arms, you're gonna don your +Def gear and load your Dodge and Parry just like a Prot Warrior. You don't get all the fancy tricks like Shockwave and Damage Shield. But what you have is more than enough to Tank a Wrath 5 person dungeon. I wouldn't recommend it with the average PuG, but a good group that can stay on target, cc properly, and mind thier threat would work great.
Give Prot a try. And give Arms tanking a try. I think all Arms Warriors will enjoy the challange and the experience. Every skill you have will make you a better warrior. Tanking is a great one to have... and it's all kinds of fun, too! :D