The single player campaign starts off with you boarding a ship with the rest of your squad. You blast off to head for the Strogg planet, just moments after the 'lone soldier' kills the strogg leader and takes out the planetary defesne guns, a la' Quake 2. The ship takes a hit and crash lands, and you awake in the middle of a fire fight. The game does a good job at creating a realistic atmosphere. You can see huge walkers in the distance obliterating other marines, smoke eminating from some unknown wreckage in the distance, and theres burts of gunfire erupting occaisionally. You get introduced to some of the other members of the squad and participate in a few scripted events. This style of narrative, interactive gameplay involving a cohesive squad reminds me a lot of the feel of a more interactive Call of Duty, which is definately a good thing. I have heard a lot of bitching from reviewers about the enemy AI, but personally I was very pleased. The enemies would duck back for cover if any was available, and different enemies had different tactics. Things you were familiar with from Quake 2, with some cool addditions (the guy with the blade hand can now roll towards you at a faster speed when in close range).
The graphics are great, are more varied and, thankfully, more well lit than Doom 3. The models look fantastic, notably the strogg. All the animations looks wonderful and are very fluid. I very much believe that having the realistic shadows add a great dynamic to the gameplay, as you can often times see an opponent coming if you're observant enough. The environment looks great and manages to 'work' very well, albeit its not quite as intricate and Doom 3, which I think is good for faster, more dynamic action. In multiplayer the graphics are also very good. The levels, as you can tell from screenshots, are not incredibly detailed and flashy, which again, does well to gear the game towards fast moving, fluid action. You simply dont have time to walk around looking at shiny things.
From the short time I got to play the multiplayer, it felt very much like Q3 to me, even having not played it much myself. It was fast, theres a lot of movement tricks involved, and the physics felt the same. It's not quite as 'all over the place' as Q3, as it seems like much of the 'push' from splash damage weapons has been toned down. This is going to be very familiar territory for all of you whom have been playing Q3 for a while, which I think can only be a good thing (if it isn't broke, don't fix it).
I think this game is going to be a fantastic platform for mods. The MP possibilities are already well known, i'd guess, but in particular I think people will do great things with the SP possibilities as well. The 'squad' AI, and its ability to interact with the player, as well as having weapon mods, provides a pretty awesome base from which to create from.
To sum up, I would definately give this game a 9/10 without hesitation. The gameplay is tried and true, and will be a hit with people who like Quake, and what it stands for. I've enjoyed every aspect of the game so far, and find it a very successful followup to a fantastic series. Viva La Quake!
Anyway, those are my impressions as a n00b. (And first time poster..

-edited for corrections-