Nice info. Not having had the time to test this out myself, I have learned one or two things from above. I have been playing on hard as default since I started 2.0 but I guess I'll continue on expert for now.
Certainly there are some characters who's special seem a lot more worthwhile than others. In the playset I felt that Cap America's was decent so he's one of the few I've maxed the special on so far. I have not reset any skill trees as yet so I'm always open to some tweaking here and there. In saying that I have a lot more characters to get to level 20 as well. And given the large amount of characters, I'm also somewhat open to having characters that are not absolutely 100% optimised, if it means they play uniquely compared to others (i.e. Nick Fury's body double come to mind as a fun alternative hit and run play style).
Looking forward to hearing more details character-by-character. And I would be happy to add my two cents where I can.