Stranger things have happened, but i have trouble imagining it turning out well. A lot of the fun of the game's combat is the counters and effortlessly killing them with a well-timed counterattack. That probably wouldn't translate against another player, because both would probably just wait for each other to attack so they could counter it and nothing would happen. Of course, they'll probably have to change things up to accommodate that, but I don't see it working out well.
The 'tag' like gametype looked like it could be pretty fun, but I would also want to get into big brawls with a lot of people. As Transience said, a big part of the combat depended on counter-attacks..so I guess it will really depend on whether or not they can make the combat fun for player vs. player and not just player vs. computer.
The AC Brotherhood feature is really much like the single player in desing and function controols but with a option to choose your charcter and special abalities http://assassinscreed.uk.ubi.com/brotherhood/#/videos/Video/7827