So I had the chance to expose myself to a little extra Halo (I totally wore a trenchcoat), and I have some updated thoughts.
Firstly, this is a game that really, really doesn't screenshot well. Nearly all screenshots look universally like ass, but the game is much better looking in motion. Now that's at least moderately common, but I think it applies particularly to Halo 3. Not because the game looks like the best thing ever in motion, but just because it looks really lousy in screens. I think on the whole it's a pretty good looking game, actually, it just has several weak points that sort of stand out a bit and bring the rest of the visuals down a few pegs. Environmentally, for instance, there are some jarring areas of geometrical simplicity that look pretty terrible, whereas there are also some really sweet-looking areas like K-Man said, that actually make you want to stop and look at them. They should have just worked harder on hiding seams and stuff. Models too suffer from this sort of design. On one hand, they animate decently most of the time and have really awesome shadows (the game seems to be shadows really well), but on the other hand, sometimes the lighting seems to show them as more simple, which kind of makes you feel like when they do look really good you're just being fooled... and some of the animation falls below the generally higher standard. Another area that I think looks much cooler than I first thought is explosions. A lot of the crazy energy weapons and grenades and stuff have cool effects that look good, and some of the vehicles seem to spark and fume in pretty cool ways.
Anyway, I'd still say this is a better looking game than it's getting credit for on the whole, it just has a hard time hiding some of the weak points. What's good looking is very nice, it's just those little things that get in the way a bit.
Also, whereas all elements of Halo 1 and 2 art design bored the living fuck out of me, some of the vehicles and stuff from 3 are actually awesome. I didn't expect that so much, as I'd only seen a little bit of that outside the game... but yeah, these look really cool. I hate the way when, say, a guy is riding one of the open-topped things and itgets flipped, and you can still see him sitting inside the model as though nothing were happening... even though his ass should be getting pummeled to death as he rolls his vehicle down the side of a hill... but that's about the only complaint. Otherwise, a lot of the stuff looks really sweet, at least design-wise. Vehicles also look a lot better in motion than in screenshots, because in motion the low texture res doesn't stand out so much, and the other elements of their design seem actually pretty nice. This is the first Halo game that I think has done something good artistically, so that's definitely a plus over the previous 2 which I found utterly bland. 3 definitely adds some flavor.
Gameplay seems about the same just with much, much better environments in which to do it, which actually helps *a lot*. That to me was the biggest problem with the first two by far. Other than the art issues and level design issues I thought the games were fine, but I felt like those two areas just killed things utterly. Given that Halo 3 manages to fix that stuff to a pretty good degree, I suspect the game to be a lot more fun to play on the whole.
The sacrifice is, of course, the super-super-super short campaign, which is really a shame, because with a little more meat to it, I think the game could actually have turned out awesome. As it stands, I just can't fucking believe anybody could give it Editor's Choice and scores of 9+ based on the severely short length. Again, if it did a few more amazingly creative things maybe it would be forgivable, but it doesn't. If it had a little more beef to it and a couple more hours, you could forgive it for being a little short, but... it is what it is.
Anyway... from a die-hard Halo unfanboy, I can say that 3 definitely looks like a huge improvement over the other games in all ways. It's just a shame that there isn't more to it and that they didn't try to shake up the formula a little more. It definitely plays like Halo 2, which is fine because the level design is so much better, but it could have stood a few more improvements.
So yeah, take that for what you will. I was able to see the game at somebody's house for a while, and so while obviously my thoughts aren't totally comprehensive, they're at least a lot more positive than they'd been before. I still don't think Halo deserves the endless popularity, and I never will, but if the other 2 games had the apparent quality of 3, I don't think anyone would care because they'd be having fun playing along with everyone else.