Unbearable cut-scenes; slow and stiff gameplay. There is so much potential, but wasted on tedious cut-scenes and limited gameplay. Many flaws in the stealth and AI as well.
The voice acting and script is embarrassing! The characters are uninteresting and just plain awful.
I haven't played MGS5 yet, but I believe that the open world and new gameplay will make for a lot better game.
As for the PS3 system - I recently played Splinter Cell Blacklist and it is lightyears ahead of MGS4 (in every possible way)!