Join the club... there's stuff you can learn reading ingame books, and the basic tutorial. They will implent a better tutorial, but so far, this is what we got. I too still wonder about many things, but I will try to answer your questions (and potential future ones).
* Exclamation mark over characters mean you can talk to them for some extra dialogue about what just happened, and sometimes even additional trait points! The first XP thing (A) says amount you have, the second amount (B) needed *still* for the next level (so it's not B-A = XP needed, A rises, B lowers till a levelup). I know, I got confused there too at the start.
* Leveling: Yes. Some stuff updates automatically too of course (like Hitpoints). Depending on what level up you get different bonusses to spend on improving your character(s).
"I assume it requires 2 ability points to upgrade an ability that's already at level 1?"
* Yes, that is correct. 2-3 requires 3 points, etc. Note that means base. If it's saying "1" from an item or trait, of course it will take 1 point to go to 2 (1 base + 1 bonus)
* Hover over all stastics in the inventory to get some idea of what does what. Not everything is too clear (like how perception improves crit. 6 gives 2 anyway, or how 'perception and speed' total is calculated), and some things aren't shown (like lower magic cost for +int), but it should give a fairly good indication what to go for, especially the Action Point mouse-over
