It has kart racing. Kart racing! With adorable chubby faced characters. Valve has finally lost it—in a good way, mostly.
I wasn't kidding about the bumper karts. I would never kid about bumper karts.
There are also crazy mid-match effects like shrinking and the level filling up with water, over-the-top items like the necro-crusher hammer, and of course, ghooooooooooosts. There's a guide to all of that here.
Players are finding all sorts of other weird things nestled beneath the Halloween tree—which, as we all know, is a spectrally gleaming dead pine salted with the ashes of deceased loved ones and wrapped in chains (or, in a pinch, belts from Hot Topic). For instance, there are...
New cosmetic items and effects:
And a bunch of easter eggs and jokes, as cataloged in this handy album:
Unfortunately, it's not all scrumptious nougat-filled fun. Players have noticed, among other things, that Carnival of Carnage doesn't play all that differently from Doomsday, the map on which it's heavily based. Moreover, there are a number of glitches that have popped up, both during the more traditionally TF2-y portions of each match (read: blasting each other into colorful cartoon giblets) and in kart segments. There's even exploits that trap people behind glass doors and allow karts to fly.
Still though, it's at least a shot at something different, even if it doesn't entirely succeed. That's pretty neat for a game that, as Luke pointed out, has far outlived anything else even remotely like it. To be honest, TF2 is its own thing, its own ecosystem, removed from the cresting and crashing waves of other multiplayer games. In part that comes from Valve's dedication to going out on a limb, repeatedly, over the years. Do players always dig what they do? No. But the game is open enough that players can make their own stuff out of whatever Valve adds, so it's not the end of the world. Team Fortress 2 lumbers on, an eccentric, wizened, confident beast.
Kinda like a Dracula. Or a really smart, probably college-educated mummy. I don't know. Something something Halloween.