I see a problem with skills stacking - unlike DD, there are more than 10 skills in each category, so stacking them up like that would cover most of the screen. DD UI is not really an option that way and stacking is done in skills menu anyway, there just gotta be an option to use them in combat as well, instead of dragging them into a hotbar.
I still believe that hotbar is the best way, but it needs to be elongated to fit more skills. And good call from Mikus for suggesting larger icons for hotbar switching.
As for the key mapping, I don't think that all skills are in need of mapping, just most frequently used. Meaning 1-0 keys should be manually mappable to the preferred skill.