Warning: Spoilers
*VERY small spoilers*
After GTA IV and Red Dead Redemption I was unsure of whether Rockstar (alright, they didn't actually 'make' the game but you get the idea) could recreate another setting in another era in such a high quality game. And although the game mechanics have change minimally, there are two main things that this game has that makes it brilliant.
The first of these is the facial recognition software used. Rockstar pioneered a new technique whereby 32 cameras are used to capture the faces of their voice actors, and you can really tell in the game. Characters' mouths actually forms words, they blink unlike robots, hell, you can even see them sniffing at times.
This leads on to the second defining feature of L. A. Noire. A lot of the game involves finding clues and then using these clues to interrogate suspects, where the facial recognition is important as you will need to tell whether your suspect is lying or not. As far as I'm aware this is not incorporated into any other game, or at least it is not done so well or is not implemented on such a high scale, and that's what makes it great: it's new and different, a feature that is rare in gaming these days with the endless slew of FPS and driving games.
That's not to say there isn't any of the 'go here, kill him' missions everyone has come to know and love from Rockstar, there are just fewer of them. The cases you have to solve aren't repetitive either, each one involves different people in different circumstances (except for a few in the middle of the game, though those are just as gripping as the victims of the crimes all have exactly the same M.O).
The game is long and packed full of things to do outside the main story line - it ships on 3 Xbox 360 discs because it's so huge - and you won't get bored with street crimes to solve (side missions), cars to collect and tourist attractions to find. And with lots of DLC planned, it doesn't look like a trade-in or rental either, a top game.