Barely anything new, and what is new is novelty.
The graphics look spectacular at a first glance, starting the game waking up with a hallucination of this pedo guy who put you in stasis in the last game. He has all these great facial animations that make him characteristic. In fact, all the characters do, but the graphics don't go beyond that. The rest is low texture resolution with ugly shadows and cheesy effects.
About a third of the way through the game you get the Gravity Gun, apparently the most wonderful aspect of the game, is merely a novelty. It's incredibly useless until the game makes you use it, in which you move objects in order to solve incredibly easy puzzles with incredibly slipshod physics. You're much better off really playing the game with conventional weapons. But, of course, they're all generic and flat too. You can't zoom in, you can't switch fire modes, and you can't lose.
The game is incredibly easy with abysmal AI with terrible aim. Someone who can't play Doom 3 on easy can play through the entire campaign on hard without taking cover once. The only unfair thing I can think of on the hard level is a combine shotgunner, who fires off three double-barrel blasts without pumping his gun like you have to. Of course, that's just a bad programming error with Valve to fault because they don't understand how their badly modified Quake engine works.
The story is practically nonexistent except for the incredibly basic premise. Aliens have taken over the world and you have to stop them. And of course everyone knows who you are even though you were nonexistent for twenty years or so. The episodic sequels clear up this problem a bit, but they aren't part of this game, now are they?
The level design is kind of cool, but incredibly linear. This is a linear game with only one outcome in each level. You go through gigantic cities, large coastlines, and dark and (very un-) spooky villages with every route except one blocked off with some kind of wood barricade normally. And even though you can destroy a dock with a crowbar, you can't break these down. Somehow, you always manage to get to the right place that the mindless followers you have tell you to go to.
It's good for all the mods, but they're only created because of the amount of advertising for this game. There are tons of better engines out there with better graphical and physical capabilities, but they're almost completely ignored.
Get this game if you want a short collection of engine tech demos and eventually a few neat mods.