click the right mouse button once for ADS, click and hold just to zoom. You can also click and hold to zoom while ADS. Also, if you have the 12x ACOG scope click ctrl+D to cycle to the 1x holo on the top of it and back.
Once I got used to how the combat worked I preferred it to almost every FPS out right now.
Teekey - I have 1 key left if you want it, just send me pm with your steam id or email.