It might be a noob question, but I have no idea how threat works in D3. Is it dependent purely from damage? Will melee classes generate more threat? Will there be threat related skills? As a wizard what should I do not to pull agro and do decent damage?
You are stuck with a WoW mind too much. Nothing can be said about threat from beta plays. I'm guessing every monsters have AI script deciding which target they prioritize for auto-attack and skills. I think a lot of monsters just try to hit the closest character with auto, skills just switch target often, especially if the monster can teleport in some way.
Devs have said repeatedly that all classes will take damage. Critters will have ranged attacks and they will pick your wizard to attack (assuming you actually stay close enough to help your party). I didn't play the beta long enough to get a whole lot of insight, but it seems like different monsters have very different AI for determining attack priority. I also don't think accurate conclusions about Hell and Inferno mode AI can be drawn from the beta.
Your best bet will be to maintain a couple of defensive abilities and be prepared to use them.
Completely different playstyle than an MMO. No threat tables, no agro. The barbarian does have a taunt, but only about 1 in 10 barbarians you meet will likely use it. Odds are, the barbarian and monk will take the bulk of the incoming damage in a party, but you really can't reliably count on that to keep alive as a glass cannon. What you will want to rely on is your own skill and tactics beyond the Tank, heal, DPS mindset.
i have played with mah wife (unlocked her from cellar for public test) and can clearly say at least for fat yellow mobs and sk so they locks on whoever damages em hard but can change target if one ran away for long time and other player is in range. They wont change target if one still hits em hard or if other hits not hard enough.
Please have your "wife" (although, it's probably just a rubber fist you keep under your bed) smack you in the face with a dictionary. Have yourself castrated while you're at it, we don't want your kind to breed.