Review from a veteran GT player, playing since the original in '98
First off, what's new? One of the very few complaints about the GT series as it went on was much the same as every other thing people complain about when there is a sequel to any game - that it's not very different from the previous game. Well, to be honest, things are much the same with GT4 in terms of gameplay. You start off with low credits, buy the best used car you can, start the Sunday Cup, build up cash till you have more money for a better car for the professional races, then if you really want to go 100%, by then you should have enough cash/cars to try the manufacture events or endurances. Sound familiar? Yes, maybe every GT has been like that, but I say - if it ain't broke don't fix it. That style of gameplay made GT what it is today, and I'd be dissapointed if they changed the core of the game.
So, still, I havent answered that question about whats new. Well there are over 700 cars, more than any other GT has ever had, more tracks (still includes all the classic originals of course), and more music which in my opinion is the best GT soundtrack yet, and you can of course select what music plays while you race. The graphics have improved somewhat, hard to say exactly how though, since GT3 looked so good it was hard to improve on, but there's definitely something different. Also some new features (finally!) like a photo mode which looks stunning (you can picture your car at any famous landmark in the world, as well as on any track, and print it out), "B-spec mode" which is excellent - it allows the computer to take control of your car for the race, which is very handy if you're just lazy or if you cant be bothered with really long/endurance races. You can also speedup the time it takes in B-spec mode, cutting down on boredom of waiting/watching (you put B-spec mode on, leave you PS2 on for a few hours, come back and you 've won the race!) You of course need to set up your car carefully for this, but if you know the game, its no problem.
A few other things I can think of are that are new are the 'driving missions' which are pre-setup up challenges like having to overtake a car half the track ahead of you and you only have 1 or 2 laps to do it. Challenging and rewarding if you need a break form the usual gameplay.
There's probably a couple of other new things I just cant think of right now...
Oh yeah the way the cars handle is even more realistic. Some people complained this game was too realistic but thats the whole point - its a 'driving simulator'. And with GT4 the driving physiques are some of the most realistic I've ever seen in a video game (I've been driving along time in real life too, I do have an idea of what I'm talking about)......you might find them hard to get used at first, but when you do, the new handling is better, if not as fun or 'easy to drift' as in previous GT's.
Overall if you're a fan of the Gran Turismo series, or if you have played any of them and liked it, chance are you'll love this game. It's been improved here and there and the amount of new cars, tracks, races,etc will keep you playing for a long time.