Welcome
I will try to answer as best I can, you might find some better detail in the wiki.

1. you can plug in into and it mounts the SD cards as removable storage, flash drive like. This is slow for me though. There is also a SAMBA addon that would let you share stuff over wireless with that.

2. there is handwriting recognition but many say it is sub-par, I personally find that it is just the same as a palm, but then again I am was never good at writing consistently.

3. After the new updated OS comes out(any day now) it has been shown to play youtube files just fine, there are also apps that can d/l videos and store them for latter.

4. The N810 is new it looks a little nicer has GPS(built in), slide-out keyboard(if you want that), and only takes 1 smaller SD card, as opposed to the N800 that has 2 full SDHC slots, O and they say it has a better screen for sunlight viewing(I find my N800 just fine,not supper bright but ok)