Achievements are lame.
Sarcasm aside, everything good you can get out of an "Achievement System" you can get out of two REAL additions to the game (both of which are planned):
To be honest, the use of achievements in TF2 to unlock stuff irks me to no end as well - with the medic, then pyro, and now recently heavy classes having updates, when medic first came out, it pretty much ruined the game for me, only new maps kept me interested for a short while but the game became imbalanced IMHO. Now with the heavy out, it does seem to be self correcting to a degree (as in, I don't see the entire server full of heavies even when it was initially released).
I hated the Medic and Pyro ones because some of them were outright laughable. Burn a spy when he flicks a cigarette? That'll never ever happen in normal play.
The heavy ones though, those actually led me back to the heavy and made me realize what I'd been missing by playing other classes. There are almost no "never achievable during normal play" achievements for the heavy, sure, some are a little long term, but they aren't impossible. I got 21 just by playing. No I don't feel super special or anything, but I think they actually made me a better player (firing 1000 bullets without dying is hard, but if you can do that every life, then you're doing something right).
On the other hand, ran into this bitch (medic) who wouldn't take advice on how to keep the heavy she was ubering alive longer because "[she] might die and lose [HLStats] points!" Oh my ****ing god. Oh my god, points, lose
30 points (of 14,312), whatever shall she do?
She told me I was a moronic noob because I thought I knew how to play the game and only had 7,700 HLStats points, had the audacity to tell her how to play the game, and she'd never once seen me before on that server (or any other, but the stats points are local to the one server). Mind I'd logged in 4 hours prior with 6400 points, that was a good day for me (a week prior I'd been hovering around 5200 points, never rising, never falling, just gaining back losses and losing gains--then I climbed 200 points in a day, then 400, then 600).
Achievements are stupid in general, but some I approve of. If they make a player play the game better then the achievements are good, if the encourage the player to cheat "just to have them" then they're bad.
Medic made me cheat. Pyro made me cheat. Heavy made me better (and god, I love the sandwich).