Mobs AI is lame, but i suppose we can't expect better, it's a common flaw in most games. At least, they should compensate this with higher mob damage and hp (in hard difficulty).
As someone who has also played more than 30h on the alpha/beta, mostly in co-op, I can say that all your points are indeed very true.
I'm not too concerned about weapons and skills balance, because I figure that must be one of the latest things to be done down the development pipeline and that it shouldn't be too hard to correct that course by altering some numbers... but I also share your worries about the ennemy AI. True, we haven't had the chance yet to encounter some higher level monsters whith a greater set of abilities, but the current ones do not offer much challenge.
It's indeed very easy for the players to alter the battlefield and stun lock mobs or damaged them over time while easily escaping their pathetic attempts at closing range... and the AI currently reacts poorly to those situations. You also rarely find yourself hindered by the same spells you cast relentlessly at your opponents, which makes it really unfair, to say the least... :P I really hope Larian has some surprises in store for us in this department, because it would really make a huge difference in the final game. Right now, combat is still fun but becomes trivial once you reach level 3-5.
