The game was great, but not perfect. I think my main gripe is how R* seemed so focused on realism/immersion that the gameplay suffers as a result. Other people have touched on this in detail but I think the most egregious examples are the animations. It's cute seeing the drawn out looting/skinning animations the first couple times, but it quickly loses its appeal. The movement in general is really slow, and not in a good way.
With that being said, Rockstar did a fantastic job on the graphics and world building. Even though the map is way too big for my liking, it's breathtaking to see the sights. One of GTA V's flaws was how the desert/forest areas were pathetically tiny, but Red Dead actually got the scale of the non-urban areas right.
Story was great for the most part. I very much enjoyed Dutch's and Arthur's character arcs. It was great seeing Dutch regress from this noble, charismatic leader to this irrational shell of a man watching everything around him crumble, partially from his own doing.
Arthur as the protagonist was a pleasant surprise. I expected him to be generic but I enjoyed seeing him transform from the loyal muscle to a sick man trying to right his wrongs before he checks out (I did a high honor playthrough).
Not sure if Micah being the rat makes much sense. Seems weird that they would reveal it to be Molly, but then make Micah the 'actual' traitor. If he started snitching after the Guarma stuff, then how did the feds foil the gang's plans all those other times before? Didn't seem well developed, but could just be me.
Ch. 6's ending was just beautiful, in a tragic way. The entire "Red Dead Redemption" mission was perfect. I stuck with the Valentine horse the whole game so seeing Arthur comfort it as it died was extra heartbreaking. The fist fight sequence and Arthur's entire ending monologue was amazing.
"I tried. In the end, I did. *wheeze* "
I actually didn't care much for the epilogue for some reason. On paper it's cool to play as John again, but I found the missions to be a bit underwhelming. I know it's meant to bridge the gap from RDR2 to RDR1, but it sorta felt like un-needed fanservice. I think I felt this way partially because the story had you jump right in as Marston without even giving time to digest Arthur's death. Maybe if the credits rolled first, and then have the timeskip happen, the transition might have been pulled off better, I dunno. The last mission was piff at least, especially when "American Venom" kicks in as you fight up the mountains.
It's also weird how they remastered most of RDR1's map and basically did nothing with it. The map is big enough as it is without including the New Austin area. Ideally, Rockstar is going to remaster all of RDR1 and sell it as DLC, but seeing what they did with GTA V's SP DLCs, I'm not holding my breath.