All right, This is not only one of the best games ever, Resident Evil or otherwise, but it is the most overtalked about game in the entire world. The only games that have gotten this much attention are Guitar Hero, Metroid, and Diablo II.
RE:4 is different from its predecessors right off the bat, because it uses live action sequences in the GC version and FMV in the PS2 version(to save time and quality, but it takes a bit longer to load them.).
The game also uses an over the shoulder camera, though the original workings of it did have times where it had the old static camera. Though it really is great for aiming, its at an angle so aiming straight is actually a little left than it would be. There are also times where the camera can hurt you, which is why its a good idea to have a stereo-capable TV to know what around you
Also, the zombies are gone, and replaced with brainwashed, parasite infected cultists. These guys don't want to eat you, they just want you dead. They can use weapons too, so watch it.
A new aiming mechanic was added, thanks to the power of the systems. Now, you can aim for the leg and they'll trip, or aim for the head to stun/kill them.
Most of the game takes place at night, and there are some parts that can get to you at times, like the novistadors. These humans experimented on with the Las Plagas(the parasite, and spanish for the plague.) and turned into bugged freaks. BRRR!
People have bitched about which system its better on. Honestly, it all depends on which system you play it on first to like it. The GC has shorter loading times, a higher polygon count, but Leon S.K.'s hair is blond. BLOND! also, the game is on 2 disks, with one lvl on an ENTIRE DISK! however, the PS2 version has more goodies at the expense of a lower polygon count, one disk, longer load times, and a shallower pixel fog. But, you get stuff like a new mini-mission, starring Ada Wong, and a costume for ashley that makes her invicible and incarriable, making pro mode much easier.
And, if you beat both modes in the PS2, you get access to the mother of all weapons: the PRL-412. this nifty Laser can defeat ANY enemy within two hits gaurenteed! for bosses, you can just flash them and they die. though it is very big, it will become your best friend.
In short, either version is great, so just play it on whichever system you have. If you have both, your choice, if neither, buy one of the systems, or go to a friend who has one of the systems and play it. You will love it. and to all those that say anything else, YOU ARE A BIASED GAMER AND SHOULD BE HUNG AND QUARTERED FOR YOUR OFFENSE IN THE IMAGE OF THE GAMING COMMUNITY!
Seriously, stop talking about Resident Evil 4. Its two years old, for Christ's sake. It should already be out of style. It never revolutionized anything except the RE series. You don't see anyone still talking about Super Mario Bros. on a daily basis, do you?