I like some of the ideas posted here. Playing a lot of Team Fortress 2 I'm used to playing for achievements and some extra wouldn't hurt.
The existing ones are mostly pretty standard and/or dull (get all xp in every level, complete the game, etc... not really that original).
I like achievements like the Buddhist one suggested above (although that is already in the game you know
"undead have rights too"). So while some of the existing ones are nice, extra's would be interesting.
Taking some of the ideas above, there are some seperate "classes" for the achievements I think. Some reward you when you do something good
(play out a level, kill x enemies, ...), some reward you for doing something in a special
way (playing without killing, Robin Hood one above (very nice btw), etc. ) and others "reward" you when you f*ck up
(die 25 times in 1 level, drown 40 times, get hit by a fireball x times, ...). These last ones are especially interesting for new players I think, because even if it doesn't go that well, you'll get a morale boost from the achievement
There's no need to make them TOO exotic I think (like "can't decide" or "mass flying"). They shouldn't mostly be things you wouldn't get if you didn't really play for it, but things you can also get "accidentally" by playing "normal" (Robin Hood is yet again an excellent example).
Also interesting would be : co-op only achievements, so you HAVE to play co-op to get them. (of course this will get some reaction from people who won't play co-op, but the co-op players out there will greatly appreciate some original achievements).
Some achievements i was thinking about myself :The 'ol switcheroo
: kill 3 successive enemies with a different character (i.e. first one by wizard, second by knight, third by thief) maybe within a given timeframe or something like thatWatch your back
: attack x enemies from behind (always nice to jump over the shielded ones as knight to do some ass whoopin')
: (co-op only) : Every player needs to kill the same amount (or off by 1 if there is an uneven amount) of enemies in the level (would be quite challenging to keep count I think ) (Legolas vs. Gimli for the LOTR fans
I really hope there will be some updates and some extra achievements, because if there is anything Team Fortress 2 should have thought game developers, it's that continued support really makes a difference