Wow, I'll try to answer as many as I know:
3. For some reason (money I'm sure) AT&T limits ring tones to less than 300Kb. In practice it seems like this is about how much of a song you'd hear before the ring repeats anyway (depending on the phone?), but it also means you'll need to edit most songs to pick out the best 30 seconds or so. I think I've used Audacity for this. I'm sure there's a how to on howardforums.
5. I took the memory card directly from my Samsung and it had no trouble with any of that media. I also have a 3.5 Meg file set as my a pic for one of my contacts, so it doesn't appear to be a problem. RAM might be a problem for larger files, but I use Nokia Image Exchange and haven't had a problem
6. Yes I am able to open .txt files
7. Yes gmail app works
10. I think it played an .avi from my samsung. I can probably check this later today
11. I'm not aware of a flight mode; could be there
12. Yes the record app is included; there are 3rd party apps which do a better job of managing this
13. I also think this works, but have not tried it.
Several of these items might be allowed as standard or extensions to S60 so you might search for apps for S60 that fulfill your requirements.