Multiplayer levels are pretty incredible, and almost all are very large. Vehicles litter the maps--much like UT2004. Although it's a challenge for a small group to bring down a Warthog filled with even numbers with just machine gun fire alone, on some maps there are placed energy canisters (think exploding barrels). I haven't seen any ridiculous physics like in Halo 1 yet, but I assume they're still in there. That's not to say that Warthogs don't get launched in the air when hit with explosives (or bodies flying through the air if they didn't get out of the warthog in time before it goes ciritical and blows up).
The fabled plasma sword is either kept in an out of the way area (not hidden, just far away from the main action) or in a centralized, easy-to-see-but-hard-to-get-to location. I still haven't figured out how to get it to do one hit kills (I've killed people in one hit, but that's been because they've been slightly damaged before). I've tried holding various buttons down, but it doesn't seem to do anything different than the two animations for slashing. However, it is a pretty quick weapon, so it's not hard to flick off two swipes (which WILL kill ANY enemy, even an overshield protected one). The challenge is maintaining a close enough distance to your enemy (charging head-on at someone to try and get a quick kill gives them a nice big target to stick a plasma grenade to). However, if you can manage to get stealth camo and sneak up on someone...
Ghosts and Banshees have new L-trigger functionality (boost). The Covenant Mortar Tank (for which I can't remember the name right now) is also included on many maps, and it too has a boost option, which gives it some much-needed mobility. Ghosts can get up to some pretty high speeds now, so expect lots of bodies flying as n00b pilots try to master the controls.
One of the coolest aspects is the ability to jump on a moving vehicle and forcibly eject the driver out. Run after a ghost or time your jump right at it and hold X and the camera will switch to 3rd person as you see MC (or your Covee Elite) hop on the back. X again (or it may be A I forget) will kick their punk ass off and you'll take the wheel. This ejection won't kill the other player (unless you kick them off a cliff or into some other nasty place). Not sure if it's possible for that player to deny your attempted boarding with their own button presses. Suffice it to say, I think the coolest experience yet has been jumping off of a high level with perfect timing to land on the back of a Banshee, kick the driver out in mid-air and watch him fly to his death.