Some spoilers below.

There is a huge amount of exp that you can lose in quests depending on the choices that you make. For example, Persuasion in particular can lead you to lose out on a large amount of exp because failing many of these checks means you usually need to fight a bunch of dudes. I have finished the game three times now. I believe I was 21 at the end of my first run but 22 at the end of my second and third runs. On my second run, I hit 22 with relative ease before the final fight because I did not have Persuasion and had to take the long-winded/combat approach to many quests. On my third run I probably covered the most content but missed out on a bunch of exp because I had a main char with Persuasion and was able to bypass the checks. For example, on the Nameless Isle, if you kill Alexandar and persuade his spirit to give you alignment for each of the gods, you can no longer obtain these by praying at the altars, thereby missing out on a large amount of exp.

Mind you, I also missed out on a bunch of quests because I did not take Pet Pal (had it removed by Hannag in the first run and did not bother with it in my second and third runs because I was playing Lone Wolf).