I'm not sure why, but for some reason the amount of content and different gameplay mechanics in this game is more difficult for me to digest than TW3 or Fallout 4 (from what is known so far, anyway).
I have two main concerns. The lesser one is that PvP will influence players who aren't interested too much. The bigger one is that, like many open-world games, it'll have far more filler material than interesting one.
Let's take TW3 (shocker) for example. On one hand, it has a huge amount of quests. Most of them, mechanically, are repetitive, but they still kept me engaged thanks to their writing and their stories, and sometimes the choices I had to make. That's an advantage of the RPG genre. On the other hand, there was a bunch of bandit camps, hidden treasures and loot caches which I couldn't care less about (and it doesn't help that almost everything there is obsolete due to the witcher gear). I worry that MGSV will have a huge amount of the latter.
Either way, it looks amazing.