I think an interesting idea to combine achievements and exp is have achievements increase a modifier to exp gained.
For example, a fresh player will get 1.0x exp per game.
A level 15 player will get 16x exp per game.
Alternatively, a player with 2 achievements will get 1.2x exp per game, etc etc. You could also make each acheivement rank in a given type give more (for example, first achievement gives +0.1x exp, second gives 0.11x exp, etc).
This way players can decide for themselves whether they want to play lots of games doing what they wish or spend some time getting an achievement to increase their xp efficiency.
Of course there will be min/maxers who go for achievement spam but that is how it is in every game.
I will echo the sentiment that I like the current achievement system works, but the achievements need to be tweaked. Examples that really stick out are the rebuild <5 second ones, the long range spot/kill ones, and anything that promotes "incorrect" player behavior (such as the one to extinguish fires w/ chem spray...instead of preapplying it). Many of the pilot ones are very convoluted too.
