Achievements don't add anything to the gameplay.
I would rather add chat before I think about adding achievements. You could argue that achievements add replay value to the game. But it really doesn't. This isn't a single player game with a collectionist mentality. The replay value comes from the variety of classes, different maps, and later on in-game chat so people can talk about cool stuff they did during matches. Multiplayer games should be played on the merit that it is actually fun, instead of trying to add things that you have to grind out.
Keep in mind that this is a one man dev team. I don't have time to spend on coming up with achievement implementation, nor do I want to outsource that job to somebody else.