After having spent over 20 hours with the multiplayer (via LAN and Xbox Live) and around 5 hours with the campaign, I can say that the mechanics are very different from previous games, if only because of the balance. Melee attacks feel more powerful, the x-button items add strategy, the levels work really well, and all the new weapons, vehicles, etc. add a ton. Infection is an awesome mode that we played (with eight people) for hours, and Forge is a great way to kick back and have some non-competitive fun (and I'm sure we'll be using it to make cool variants).
It doesn't matter if this stuff is original or not (most of my friends were noticing the same Unreal Tournament vibe that GP did); it's still miles ahead of anything else on console.
Multiplayer is a huge aspect of why this series is popular, but it also has a lot to do with the polished presentation, both in controls and the world. Even if the levels are very linear, the worlds Master Chief explores are huge and gorgeous, with extremely long draw distance and sharp details. Playing single player Halo, from the first game to the last, has always been fun for me almost entirely because of its beauty.
I used to play Quake pretty competitively when I had a PC that could run it (98-99, I think), and I dabbled in Unreal Tournament just a tiny bit. Though I can see the improvements in aiming with a mouse and keyboard, a controller feels more physical and involved, like its an extension of me, especially in a big group of people. Also, PC shooters always felt a little 'thin' to me, as did most PC games, whereas console stuff (especially on Nintendo's 3-d consoles) always had a lush, fully rendered look - PC's look generically sharp and artificial. This is by no means a knock on PC shooters, just a matter of preference - comparing Halo to any of these for an FPS fanatic might be silly (I'm not a connoisseur of PC FPS, so my opinion matters less). Nonetheless, Halo as a series has done more, more, and more than any other console FPS, and there's a reason millions of people play it, both campaign and multi.