There really isn't anything to wrong with achievements I enjoy doing them and they do promote replaying games and provide objective challenges that you may never would've attempted had they not been pointed out and been packaged with a "reward" but  for me it has ruined replaying a game just to replay it, because I had fun with it. I've yet to play a game over again this generation simply because I liked it. I always feel enticed to play it over so I can knock out achievements  I didn't get on my first playthrough and it's cool that they are available, because like I said it adds an extra bit of challenge (at least with some games), but I feel as though it has ruined playing the game for the sake of just having fun. I always feel like I'm trying to earn something now as opposed to me just trying to play the game a bit differently and being satisfied with my accomplishment of beating the game faster or on a harder difficulty. Now you are given an "incentive" to beat the game on hard and here's a 150 points or something and I feel less compelledto play games on higher difficulties now a days.
 
 It's also sort of ruined the creativity in setting challenges up for yourself. 
 For example there is an achievement in Black Ops for escaping the ship in a certain amount of time. Before achievements I would've attempted the same thing without a "reward". In Halo: Combat Evolved I would always try and see how fast I could do the warthog run at the end of the game or how fast can I beat the first level on legendary by myself.   I'm not saying you can't still be creative now a days but I personally feel less creative in my approach to games and replaying them.
 
I guess the achievements are things I'd end up doing anyway if they didn't exist but now I just feel like being able to see it as an objective has made it a bit harder to enjoy.
 
I'll also play a game and if I get really close to 100% or 100% then I just lack any desire to play it ever again. I've exhausted the game and have no desires to try new stuff in it.