I'd care more about achievements if they had some tangible in-game impact, but most don't, so I can't be bothered with them - either I'll get them or I won't.
The problem with all achievement systems is the same one you'll find on any forum that displays post counts and awards titles based on them - some people will then go to ridiculous lengths to make their numbers/titles higher by posting rubbish everywhere they can, and the fun little extra feature intended to simply be an added bonus to the main event (posting things on a forum, playing games) takes over as the driving reason people are posting or playing, because if the numbers are higher that means you're more awesome then all the people with low numbers!!!1!eleventy!!
Or to put it succinctly - it's not the achievement systems, it's the idiots who use them that annoy us naysayers.