1. Day night cycle is not DX10 only and will be avalable on all maps that are designed to have it.
2. There are no infintry. However there are battle-armor, which do have "jump jets". You can't ski with them though. Well... maybe. I think it depends on the air resistance set.
3. It will require more memory seeing as the maps are larger, but other than that it HOPEFULLY shouldn't be any more demanding than Crysis was. Some of Crysis's maps for instance would give me EASILY 15-20 FPS on high settings all the time, while with MWLL it's a bit better in that regard.