you have a good 5 on 5 multiplayer match , with your friends, it would be a hell of good time 
 
templars will have stronger brute force, assassins will be more faster  
 
and its the chaos theory team working on this multiplayer , so it should be interesting  
 
i dunno even though ubisoft IS IN FACT gonna start making assassins creed games yearly (read that on joystiq a while ago) 
i still have faith in this game, and we will probably get our Desmond storyline fix ( they wouldn't record one paragraph for vedic to say :P) they're probaly using this game to get you more attached to the characters so that you will get ready for the main event in AC3, because lets be honest, except for desmond and kristen bell , most of the future characters are kinda flat IMO we haven't even seen the boss templars that vedic is working for, and introducing them at the last minute in the third game would be a bad idea

 I'm playing AC1 now, stealth is important in the game (not crucial, just important) and it is very hard to achieve in a multiplayer setting, specially if it is not first person perspective. (more reduced field of vision).
 
I think co-op has a greater potential than multiplayer, but I can't imagine a feasible way to implement it yet.