I think it's definitely a much slower paced game than what people are used to and I feel like you have to be someone who enjoys taking their time just enjoying the scenery to really mesh well with the game. The other option is to go and commit lots of crimes in every location and become a total outlaw, that should provide a little bit more action for people who don't find the casual and relaxed feel of the game fun.
I think that ^this is a big factor in things. For some reason a lot of people have been caught out by the slow paced nature of the game. Its a game which is best enjoyed by really immersing yourself in the gameworld, don't run/rush places, walk everywhere, live and breathe the gameworld. The missus and I are over 200 hours played now in the online mode and we've barely even started single player, its been immense fun. The missus says its one of the best games she has ever played and its my choice for my game of the year this year. We walk everywhere, we ride slowly side by side (because after all, people don't run and gallop everywhere in reality), we sit and travel and live the gameworld. I use pretty basic revolvers and repeaters and wont upgrade to the higher automatic weapons because that's not how I want to play, I want more of a traditional cowboy set of equipment.
Now, of course, it should be pointed out that the game wont be for everything, for some people it will be the worst game of the year rather than the best, but that's fine. Not every game can appeal to everyone, there are plenty of folks out there who didn't like games like Witcher 3, Skyrim, Diablo, StarCraft, Civilisation and so on, games which are considered truly great games. Cant please everyone all of the time.