First item, city levels, work your way down claustrophobic streets, dodge into doorways, even hide in the beds of pickup trucks. The contrast between buildings and outdoor areas is made even more dramatic buy an "iris effect", which gradually lightens the screen when you move from a light to a derk environment (and vice versa), mimicking your eyes adjustment to light and darkness.
Second item, much more robust online support. This includes buddy lists, ignore lists, clan tags and rankings, indepth statistics, and rank and insignias(which can be used to 'form newbie' only to 'pro only' rooms). It also inclueds better voice quality, plus visual indicators of who is talking, and the ability to voice chat in the loby before a game.
Third item, a vastly improved single player experience, with more lifelike a.i., a wider variety of mission objectives, and a lot more conversation (delivered by real voice actore, including Michael Clarke Duncan).
Items 4-25, 12 new online maps, plus all 10 form the original game remixed for your pleasure.