There's a fairly in-depth interview with Kojima on the announcement of Ground Zeroes as a stand alone release. A lot of the stuff are things we've heard before, but he does clear up a few points which I think will help fans keep their expectations in check for what Ground Zeroes will offer.
- Ground Zeroes is the prologue to Phantom Pain, and it is intended to be similar to the pre-title sequence of a Hollywood movie, where something major happens to catch the audience's attention, before the real story starts.
- The main content in Ground Zeroes is largely meant to be a tutorial for Phantom Pain. He uses the exact word - tutorial, so there's no doubt about this at all. The story content in Ground Zeroes is limited to the night setting in the rain, but you'll be able to see other times of the day via the side op missions. There is no dynamic weather or day/night cycle in Ground Zeroes.
- Phantom Pain is the game which will feature the full range of expanded features carried over from Peace Walker - base building, recruiting soldiers, etc. Most of this will not be in Ground Zeroes at all. You will not be able to use the fulton recovery system in Ground Zeroes either, but there are optional captives you can rescue using the helicopter.
- The save data in Ground Zeroes will carry over to Phantom Pain, and there will be benefits in the full game for extra stuff you decide to do in Ground Zeroes.
- The reason Ground Zeroes is being released as a separate package in the first place is because the main game is taking a long time to develop due to the large amount of content. So the marketing team at Konami worked out this release format with Kojima so gamers can at least experience the prologue segment of the game earlier.
- Kojima wants to reward fans who buy both Ground Zeroes and Phantom Pain later on with some sort of content in the full game, but he hasn't worked out the details with the marketing team yet.
- Ground Zeroes (and Phantom Pain by extension) will support a secondary display device via smartphones or tablets. It can be used to display stuff like leaderboards. There have also been feedback from people outside of Japan for a map display and so on, but Kojima has some concerns about players diverting their attention between two screens, so he's still looking into it.
There're probably other interesting things I missed out on. If anyone wants to do a full translation or post additional info or corrections, feel free. :)