It's just overproduced. They worked on it for far too long to get the gameplay as good as it is and the story probably had to suffer through multiple rewrites. They never settled on a single direction and it really showed. It might be a coincidence but I felt like Max Payne 3 had this exact same problem.
I always find it fun being poor in GTA when you are forced to build up to bigger and better things. In V, you just get handed mansions and hundreds of thousands virtually from the start. Combined with not having new areas to unlock, you start to feel like you're just floundering instead of working towards something. That hurts the story in later stages.
I think they absolutely nailed some of the characters though. Michael, Trevor and Lamar are some of the best GTA characters ever, in my opinion.