Immersion is what really sets apart a good game from a great one.
The visuals are stunning, set in the old west. Rockstar paid very close attention to detail and it shows. From the dusty towns, to period buildings and houses, to gorgeous sunsets on the horizon, one can't help but feel like they've stepped back in time. The animations on horseback are a bit stuttered when changing speeds, but this is more often not noticeable.
The gameplay is outstanding. The fun, at least for me, is in the journey, getting from place to place on horseback. I often just enjoy exploring and finding random encounters or hunting. The unexpected really makes for an exciting exploration experience. The controls are superb, the gun play is fantastic and the dead eye is a good addition in single player mode. The only negative here is the walking is a bit sketchy sometimes, but this was true in GTA as well.
The sound is amazing, with old western movie-type gunshots, train whistles, period music and community chatter. This adds to the immersion experience as well.
Overall, I would give this game a 9.8 and submit it for GOTY candidate, if that choice were up to me. RDR really takes you and puts you into the experience. For fans of western culture and the "wild west" it really is an enjoyable immersing experience.