If you did not like the open play feel of the GTA games you will not like RDR. This is much more open and a lot less linear with advancing missions. Took me at least 2-3 hours to finally get into it. I'm about 4 hours in now and I feel like I've barely scratched the surface. The beginning is very slow, lots of talking through encounters and a big set of "training" missions teaching you how to do different things in the game (capturing horses and bareback riding is a fun little mini-game.)
They've really upped the randomness of encounters that you can either participate in or ride right by instead of straight forward missions. So far I can remember the following random encounters: some people sitting around a campfire telling stories, a guy sitting around a campfire with a kidnapped woman who I saved, a burned out wagon and two escaped convicts I had to chase down, multiple instances of people being attacked by coyotes, a guy who asked me for a ride back to town who knocked me off my horse and stole it

, some bandits around a campfire that shot me to pieces, a guy shooting birds that bet me to out shoot him. And there are tons of others, you'll just be riding around and you'll hear someone shout at you or bullets will start whizing by.
The game is VERY pretty. Big open country, huge sky, lots of little details like broken down wagons, burned out shells of houses, bushes, trees, etc. I think it's basically the same engine as GTA IV, but there is much less to render in terms of buildings and cars and streets around you like in a big city it lets them focus more on fine details in the open country. The soundtrack is great, very spaghetti western.
My only complaints so far is that it took me a long time to get the handle of riding a horse (ran my horse off a cliff by accident more than once) and the many intro missions before you could actually start having fun. I'm also having trouble with switching weapons on the fly. I accidently shot a guy in the back when I meant to pull out my lasso and capture him.
Bought Alan Wake also, may not try it out for a few days though.