When your faction controls Wintergrasp your side gets 5% xp boost in Northrend (and Northrend only, including instances).

However, imo Wintergrasp is extremely multiboxer unfriendly. If your side has more people than the other, they get a buff called tenacity, which scales with how "unequal" the sides are. The other day a ret pally came and did 18k dmg each to my 3 shammies, in one single hit (the whirlwind thing, divine storm I believe), due to him having a x10 or something Tenacity buff.

Also vehicle combat is really the name of the game -- one side is trying to smash in the keep walls, the other is using turrets to blow the vehicles up (and defending vehicles too). if you try to DPS a vehicle on foot you are pretty much going to die quickly -- it does ridiculous amounts of damage, like up to 40k per hit. You can "ride" vehicles, and I've done that (my main drives, 2-3 others "ride") but they don't really do anything and its kind of a waste.

My personal feeling at this point is that multiboxers in Lake Wintergrasp are really a detriment, and I try to avoid taking them in unless its off-peak and I just want to farm some mining nodes or some basic honor. At least in places like WSG/AB/AV and maybe even Strands you can turn the tide and help out, but its really hard to come up with a lot of scenarios in Lake Wintergrasp where multiboxers don't contribute to losing -- maybe at the end when the game is going to end soon and its a frantic battle at the keep doors, then its a lot of hand-to-hand combat and multiboxers are great, but pretty much everything up to that point is played more like a FPS/Battlefield/Halo/etc.