I wasn't terribly thrilled with the achievement system when it was in the works pre-3.0. Not that it's not a good idea, I was just kinda meh about the whole thing.
The last few days I've been checking my Achievements log pretty frequently. There weren't many that got retro'd. And the stats pages are seriously messed up. So I'm looking at, say, my gold per day (and then peeking at my currency tab) and thinking "Damn, I gotta run dailies."
I also looked at how lame my rep is with most factions. It's a great "hey, you never did this even though you said you'd go back and do it" reminder/nag. Plus all the stuff I actually did do, but never got credit for. My dungeon boss kills only go up to Cleefy??? Oh, c'mon. What am I? Level 20 again? :P
The neat part is, most of it can be done while doing other stuff. For example, I need to farm up lower level gear for Dark Clan's guild bank and my alts. Well, I can solo all those dungeons now, and yah, I'll be making a point of smacking the crap out of any bosses on the Achievements list. Twice.
Lately, I've been doing the three-things-at-once thing in Scholomance. Early in your Level 70 career, this is a good place to go to guage your progress. As you improve your gear, this instance gets slightly easier each time. At some point, it stops being a good measure. But for awhile, if you can solo Scholo in under two hours, without dying, you're doing pretty good.
One of the things on my list is finishing my Argent Dawn rep. I love the Argent Dawn and I'm thrilled silly they'll be appearing, as the Argent Crusade, in Northrend. But I still want that Exalted with the original. Scholo is a great place to grind out the rep. (Do Strath if you've got a couple of friends along.)
It's also a great place to pick up the odd gear piece for Neveron, my holy Priest at Level 55. Sure, she'll have to go in ST, DM, BRD, and Strath herself to finish up the BoP bits. But she'll go in pretty well geared up to start. And she'll probably level high enough to completely skip Assfire Penninsula. Blah!
I get rep, gear for an alt (and plenty to drop in the GB), and a step toward another Achievement!
Now all I fear is grinding out the VC and Gnomer runs for cloth to turn in for that Abassador title... gah!
Maybe I'll turn on the new low level quest option and run around naked with the lowbies doing quests... at least there's a story to go with the grind. :P And another step toward World Explorer!
Achievements. Genius or Evil Blizzard Plot to keep us playing?
PS Bored and Wasting Time Idea: Strip naked (no armor, no weapons, no nothin') and start the low level quests in an area you've never played. You can only wear gear you pick up from loot while questing and you can not spend any gold. Blast from the past! Call it Retro Rep Leveling. :P
Note to Blizz: Better underwear options. Sheesh!