MP3 doesn't need familiarity with the MP series to enjoy. If anything, the story seemed to be an extension of Metroid Fusion theme-wise.
I picked up a copy of MP after playing MP3 and was completely turned off by the controls after being accustomed to the pointer based shooting of MP3. I think I gave MP less than one hour of play time before setting it aside. I haven't bothered to pick up a copy of MP2 yet.
It would be much like playing RE4 if your introduction to the game was with RE4 Wii Edition.
MGS4 doesn't require extensive knowledge of the MGS universe to enjoy the game play segments although many will probably find themselves somewhat lost during the story segments since the series is so self-referential. Those looking for a casual gaming experience will likely be completely lost or completely turned off by the overblown narrative style of storytelling.
I never finished MGS, MGS2 or MGS3 (each played minimally) yet I was able to absorb enough to follow the story of MGS4, which has been the first MGS game I've played to completion after ten years of playing the series.







