While I like the system you've set up here, some of the abilities seem a little lacking. Notably, there is no way to move and full attack, and little in the way of utility options. Something that grants passive bonuses to perception, intimidate, and other fightery skills might be nice. Being able to ignore DR or Hardness is always good, as would be some ability that allows them to deal Con/Dex/Str damage.
Otherwise, I really like this. It feels intuitive, and gives some solid abilities, as well as a real reason to go Fighter 20.