Copy/pasted from my other post since this topic seems to be better suited.
Humans have the highest killing power, easy. Between Grenades, the awesome saucem shotgun, pulse head shots, etc, they are killing machines. I picked up the sniper once, not sure where I found it though. You know what's awesome? Blocking an alien's quick attack, meleeing them and knocking them down, and double barrel shotgunning their faces. Marines have the easiest time blocking/countering in my opinion so far. One button instead of two.
Predators I'm already the best with since I figured out jump attacks and such. I'm getting very proficient with the focus jumping, it's really what a predator should be able to do. Albeit, it's annoying when I start to jump, start to take off, and an alien grabs me from behind. Walking as the predator seems to be a viable option when fighting marines (just like crouching in AVP2), they don't seem to be able to see you until you're right close. Once I start hitting with the wristblades, it's more/less over for the one I'm hitting.
Alien, is weird because I have not figured out transitioning and leaping yet. Transitioning seems to be slow and sloppy, I hope it is better in the full game. I'm fairly certain they should be able to do a jump attack like the predator as they have a 'focus' mode too, but I haven't played them enough to find out.
I think stealth/trophy kills are annoying as hell, I hate one hit kills. I think it's funny as hell to get the train going, when alien stealth kills alien stealth kills predator steal kills alien and the original alien makes it back to stealth kill the first. I wish it was TDM instead of DM for the demo mode. Nothing more annoying than the ONLY OTHER PREDATOR IN THE DAMN GAME coming after you, specifically.
Next up, working on block/counter/power attack combos with my wristblades! (Haven't used any of the other weapons and am not likely to until I have perfected my style)