I've been so far impressed by the Beta. No major in-game bugs, although there are many small bugs that can sometimes be irritating (like spontaneously falling through the ground, and not being able to aim down sights).
Some interesting things I have noticed are;
-Knife is not one-slash-kill, unless you perform an 'assassination move' from behind an enemy (like in Halo Reach)
-The M203 can be used as it's own weapon (not being attached to an assault rifle).
-You can adjust the optics on sniper rifles so that you have no sights at all.
Overall the game is excellent, and this is only the beta. The problems and bugs that are there can be easily ironed out and I think the end result will be, and to quote Lostmymind, "undisputed king of FPS games"