Carbon is essentially NFS UG2 with updated graphics and less content, with cutscenes continuing the story of most wanted
If you've played Need for speed before, and enjoyed Underground series and Most wanted, then you'll be a bit disappointed with Carbon, its basically an expansion pack to most wanted, and I really mean that. So you're getting more of the same thing... but there's even less. That's the bad part.
The tracks are well designed but there is not enough road or diversity in the city, it feels much smaller then the city in NFS Underground 2 and even most wanted.
So what's different in Carbon compared to Most wanted? Car selection has taken a turn for the worse, in that there are less of them unless you're willing to "buy" unlocks from EA's site, for a dollar a piece.... which is certainly a rip-off after charging you money to buy the game! Many of the same cars that were in Most wanted or Underground series make a re-appearance in Need for speed carbon.
Not much has been added. They've added a few new gametypes to multiplayer modes. But over all, if you've played most wanted or Underground 2, you'll feel like this game is basically an expansion pack to Most wanted using an enhanced version of Underground 2's graphics engine. You're basically paying for Need for speed Underground 2, with a facelift and some cutscene inserts they tacked on to connect NFS Carbon with most wanted.
There is really not much new at all in single player, you have only 4 basic standard races: Circuit, Drift, Sprint, canyon, standard boss races, and Checkpoint (formerly: Toll booth races in most wanted).
I do appreciate that they tried to do something diifferent by adding AI partners. Blockers that can block other racers, 'scouts' that can show you the secret (and not so secret) shortcuts on the tracks, and drafters, you get behind them hit the button and get a boost of speed. But over-all they fall flat because the implementation suffers from poor AI and most races are si short you don't really get to benefit from your AI partners all that much, although they DO help in one respect, if they win the race YOU win, which is definitely a bonus.
Your AI buddies like drafters and sometimes blockers are difficult to use simply because you have to pretty much either be in an obvious position to use them, or constantly looking at the map to see if your bone-headed partner is even far enough ahead to be useful. Lastly you cannot change the types of cars they use or their parts, which will probably frustrate a lot of gamers out there.
So the 3 types of AI partners the give you are:
1. Blockers block other racers and take them out on you command.
2. Drafters run in front of you to provide you with a slingshot boost of speed when you use them.
3. Scouts find shortcuts for you and leave a nice rainbow colored trail and map updates so you can see where the shortcuts are once they are taken after the first lap.... only drawback is they reset, so when a scout marks a shortcut it no longer marks it for all races on that track.
Graphics...
The only new things are minor additions to modifying the look of your car, that's basically it, and even they feel like the were unfinished, and basically taken from underground 2. Hardly any new art work was done in terms of decals, etc... I really wish they'd open up Need for speed underground to the mod community and make it easy to import models, textures, custom decals, etc. EA's artists over time simply can't compete with user made stuff!
There are some interesting features like auto sculpt, where you can sculpt mathematically pre-modelled parts the artists have made. They allow you to have e a limited amount of control over their sizes and shapes according to the prefabricated parts and prefabbed range of parameters of the different autosculpt pieces... but....
The selection of pieces you can modify is VERY limited (boneheaded move by EA), while the selection of non-modifiable pieces is quite a bit larger.
Lastly, if you've seen Fast and the Furious 3 Tokyo Drift, you know exactly what Need or speed carbon is all about, the game DIRECTLY lifts all the same stuff from the movie Fast and the furious 3: Tokyo drift, it's insane, they have the Camero and/or Dodge charger that was in the movie in carbon. The game will feel a little weird if you've seen Tokyo drift and played Most wanted, since they lift so much material from Tokyo drift.