Halo, in my mind, is to the FPS genre as Starcraft is to the RTS genre. They both emphasize fast-paced, intense action which at its best makes for a very fun game that just never stops, but at its worst feels somewhat distanced from its cousins in their respective genres, somewhat lacking in what people (or at least myself) look for in those types of game.
That said, Halo 3 is still an awesome game; the use of equipment was brilliant in my mind, its definitely spices things up. I'm not sure what you mean by cramming all the vehicular action into the last level though, there's vehicles all throughout the game. My only real complaints are A/ the physics engine; sure, it might be more accurate, but vehicles just do weird sh*t now, before sticking a warthog would almost always flip it and knock everyone out: now they more often than not right themselves, and with little apparent damage. B/ The matchmade playlists lack any manner of heavy vehicles! tanks are especially important in levels with banshees, plus the banshees are better than ever with their fuel rod guns restored, they are unstoppable in the games I play, and there are few options to counter them. Besides, tanks are awesome, why aren't they in play? C/ the most important thing that ticks me off, is the goddamn AIM ASSIST!! Maybe I just didn't notice it in the previous games, but it is very apparent to me in Halo 3, and it has never helped me, and simply cheapens the game.