One thing I tested during the WotLK beta was the new achievement system. It isn't fully implemented yet, you can earn achievements and achievement points, but you can't spend the points on anything yet. Anyway, my level 70 warrior I copied into the beta has 88 out of possible 532 achievements, for a total of 820 achievement points. Still a lot to do if I wanted to collect them all. Which I won't do, because for already existing characters it will be next to impossible, as they would have to repeat a lot of old stuff.
The problem lies in the fact that up to now World of Warcraft kept track of some things, but not of everything. So the achievement system knew that I had done 1472 quests with my warrior, giving me the 1,000 quests done achievement. It didn't know that I had done more than 5 daily quests, as those aren't kept track of. It did know that I completely explored Outland, but it didn't know that I've been to many dungeons in Azeroth and Outland already. So the "Kill Archmage Arugal in Shadowfang Keep" achievement is still greyed out, together with every other "kill boss X in dungeon Y" achievement. Who is going to organize a raid to kill Ragnaros again, just for 5 achievement points?
I think the achievement system is a nice addition. But adding it to a 4 year old game has obvious problems. And some achievements, like gaining exalted status with various factions, aren't really worth doing just for some points and checking one more achievement off your list.