The also mentioned the Auto save, and the save state from iOS being implemented.. But they never showed it off.
I think overall this is just a small taste of what is to come.
UI, I am guessing a new UI will be showcased at WWDC. If you look closely there looks like some new scrollbars, and other small UI tweaks that could suggest a more refined UI to tie together the mashup we have goten over the years.
As for ful screen apps, I think (hope) this will just be an API developers can take advantage of. From the keynote, it didn't look like it replaced the green + button, but added to it. I may have missheard, but when the app went in full screen mode, I swear I heard him say "control click".
I am hoping full screen mode is just there incase the application you are using could benefit from it.. iWorks Pages already has a full screen mode for a while, and noone complained about that.
Overall, I am just going to guess that the keynote only showed off what they have presentable and polished...And also showed off just enough, with out giving away every thing else they are working on.