Alot of new features, but it really never took full advantage of them.
A bit more realistic - you can start tune your car, set callibrations to your suspensions, turbo, engine etc... Cars do go up to 350+ km/hr anymore, acceleration is slower and more accurately reflecting real life cars - NFSU, mx5 can beat a skyline (if your good enough), NFSU II - mx5 will get creamed after 30 seconds.
Freedom around the city - massive city to drive around in, there are cars randomly driving around to challange, there is no fixed race course in this mode, the city is basically it. Instead of choosing races from the menu, you have to drive through the city to get to them. There are secret races paying big $$. You also don't access to all the upgrades in one menu. There are shops dotted around the city to unlock and discover, each specialises in special parts - performance - engine, suspension, you get the idea, body - body kits, spoilers etc..., specials - sound systems, HUDs, tints, NOS colour, hydraulics etc...
Music - same style - hard rock/metal, r&b mostly.
New cars - why the hell did they remove Honda s2k?, well they got these large SUVs in though, fun to race with friends with these bulky Hummers
That's off the top of my head, sorry for hijacking the thread.
Full review for PS2.
http://ps2.gamespy.com/playstation-2/need-for-speed-underground-2/567046p1.html