Okay, here we go.
Let's just get this out of the way: the graphics are a mixed bag--sometimes you'll see some fantastic stuff (the way an object might be lit, a gorgeous environment, etc.) and then other times it looks pretty underwhelming. Yes, there are quite a lot of jaggies, and I've even run into some frame rate dips and some ugly tearing. The graphics impress in a subtle way--they don't beat you over the head with saturation and effects like in Gears of War--but I was hoping for a little more visual oomph.
Gameplay--I'm not a huge Halo expert, but it feels like Halo to me. Short, crazy bursts of shooter fun, some hot vehicular action, and so on. Enemy AI feels a little hollow, but again, I'm sure I'll face some harder stuff later. My teammates must be schizos because sometimes they're aggressive and kicking all sorts of ass, and then a minute later they're nowhere to be found or they're waiting for you to clear out an area on your own. Variety of weapons and items is nice, and I'm definitely looking forward to getting into some multiplayer action after I finish the campaign (I really enjoyed the beta a few months ago).
Story--no spoilers, I promise--I'm pretty picky when it comes to movies and books and all that, and I think that video games still have quite a way to go when it comes to narrative and storytelling, so (gulp) I'm going to say that I'm not exactly loving it so far. Fine, I was never into the story of the previous games, but it feels like one big cliche wrapped up in a whole bunch of cliches. Everyone speaks in some sort of cliched dialogue and some of the cutscenes (which are pretty) come off as pretty cheesy. High-quality cheese, but cheese nonetheless. I'm not a Hollywood blockbuster kinda guy, so some parts (esp. some of the dialogue) made me cringe a little bit.
Don't get me wrong, it's not bad. There were a few moments where my jaw dropped or I yelled out with satisfaction, and, at its core, the action in Halo is still miles ahead of some other FPS titles. But I guess after all the hype, and after experiencing Bioshock, Prime 3, and the COD 4 beta back-to-back-to-back, so far Halo 3 feels more...underwhelming than anything else, especially for a game that's supposed to be the closing act to one of the most popular trilogies of our time. I was completely immersed in Bioshock's world and engaged in the story, I was just jaw-drop-in-love with Prime 3's art direction, and I'm still excited by the action and impressed by the graphics of the COD 4 beta.
Maybe my expectations were too high or maybe I never really drank the Halo kool-aid so I can't fully appreciate what's here, who knows. But so far, to compare it to a movie, it feels like a slightly above average summer blockbuster movie--there are a few moments that truly impress, and if you can ignore the lack of originality or depth from the story, art, or dialogue, you're still going to be entertained for most of the way through. It'll be one of the highest-grossing flicks, but will I consider it as part of my Best of... list during awards season? So far, I'm not sure.