Okay, for me as hobby musician:
- Better soundengine / sounds, especially explosions (take Bad Company 2 as an example ;) )
- The soundtrack could be better, but thats not important. Just wanted to mention that there's a lot of great free music under creative commons license out there that could be used. Maybe less rock-like, a bit more electronic.
- The next BIIIG thing is the AI, I know it's a difficult thing in this scale, but it must be better in ArmA 3.
- I'm really glad to see that physics and animations will be improved.
- Optimized netcode is really important too, to get rid of those nasty, nasty lags.
- The lighting system is not the best at the moment, and one more thing I would like to see improved is the particle system.
- You can see a rough color gradient at some particles or the sky in ArmA 2, that's pretty bad for the immersion - I hope this will change in ArmA 3.
The last point is: Please don't add too much postprocessing effects. They make the game look very unrealistic at the moment (at a high cost of FPS). Please make them more decent / natural, especially Bloom.
EDIT: Please don't underestimate the sound, it's at least 40% of the atmosphere in a game. You should find more "adult" voices for your characters, those soldiers sound like 16 year old boys, but have faces like they are 40. It sounds bad when you get commands from a teenager ;)