"Ground Zeroes" implied that there would be multiple grounds.
With how hard and fast the hype train for Phantom Pain has been running, I'm glad I finally tried out the Ground Zeroes "demo," first; saves me some consternation about whether an open-world Metal Gear Solid would finally do it for me. (It doesn't.)
Contiguous open-endedness is a big plus for me, and unlike - say - Hitman, Ground Zeroes does a lot to avoid strict and obtuse punishment for being detected; if I fuck up and get spotted, running to cover is almost always an option. But the gunplay is unsatisfying enough to make it clear that this is not the way Kojima wants you to play. And unfortunately, when you're focused on sneaking around, large open maps become easy to get completely lost in; I spent most of my playing time slowly covering the same ground, over and over, unable to find the next mission objective.
And of course, then there are the overwrought cutscenes. Ground Zeroes is not a long "game" (having only one primary mission), and the longest part of it was when I was wandering aimlessly looking for the final objective -- but the second longest part was the interminable stream of ending cinematics. Many modern action games are rightly criticized for reducing high-octane story sequences to barely-interactive quick time events, but at least that's better than just sitting and watching your playable character do a bunch of cool shit. Every time the camera panned behind Snake during the ending battle, I hoped I would be able to control it, and my hopes were dashed each time.
I can see the appeal, I guess. Metal Gear Solid feels like it's become the video game version of Tom Clancy-style military nerddom. If you're into the realistic hardware and the fantastical-but-politically-grounded plot, then, great. I'm not, and the gameplay isn't enough to carry itself for me.