Guild Wars: Costume Brawl
Personally, I think that the format itself seems pretty good although I didn't get to play it enough to be sure. The skill bars could use some tweaking but that's pretty par for the course at this point - ArenaNet's track record on preset skill bars is not good. Tactical splitting and play skill are definitely important in this format, and the lack of skill bar building makes it slightly (but only slightly) more friendly to casual players than the usual PvP formats with their near-vertical learning curves.
For average to experienced players, the Ranger is head and shoulders above the other builds. This is a capping and splitting format, so survivability and mobility are the order of the day - and just as in GvG, the Ranger has the best average 1v1 matchups of all the choices available. The other bars are generally at least playable with the exception of the Paragon, which I wouldn't recommend.
Note that this is a very violent format, with relatively weak healing options, so all the characters pack self-heals. The nice thing about this is that it fixes the good-monk-equals-win problem of Random Arenas - mixes of most character types are at least semi-viable, and player skill is generally more important than the class selection. That said, player skill is important enough that generally the key to winning or losing is who you get on your team, which can be frustrating at times.
One more tip:
RUN AWAY WHEN YOU'RE OUTNUMBERED.
In a splitting format, picking and choosing your fights is very important. If it's 3v1 against you, head for the hills and just try to stay alive. If you can escape without being snared, you'll either be taking up a bunch of their team, giving your team numbers elsewhere, or you'll live to collapse on that 1 other guy somewhere else on the map. I see bad players fighting helpless odds virtually every single match. Keep an eye on your radar and start running before all the guys in the area have you in spell range.
If I was a betting man, I'd say that this format is a testbed for some similar stuff in Guild Wars 2.