First, check out the vids:
http://www.gametrailers.com/game/halo-reach/11394
Then, I'll be happy to answer any questions.
The new BMR(old BR) is cool, but not as awesome as was indicated.
The active camo perk is EASILY my favorite. So far, it doesn't seem like any of them are too over-powered. I just LOVE sneaking behind someone and assassinating them.
Took me forever to figure out the new Covi sniper is not actually a rifle, but a laser beam(old sentinel beam, with more power).
I don't like that the Elites ONLY get to dodge. They should at least be able to get jet packs. The dodge IS useful though.
The grenade launcher is not anything special, so far. I haven't used it a LOT, but I don't like it. The Covi plasma launcher though..... OHHHH MAN is that awesome.
Sword doesn't feel like it's been changed at all. Whether that's good or bad is up to the user. Same for the shotgun. Just as devastating as before.
Sprint is lame.
The whole "credits" system(instead of XP like H3) is quite lame. All it does is allows you to purchase new aesthetic armor pieces.
It doesn't seem like they've changed the customization of the characters at all. You can buy visors for the helmets. That's about all that's new in that area.
They still don't have clan support. Highly upsetting.
They say that they made the pistol what is used to be, but it still isn't up to H1 standards. Quite weak.
Let me know if you have any specific questions