My thoughts:
1) This will make it nearly impossible for there to be so many useless filler skills that everyone is using the same skills.
2) Passives might become the main reason to choose a class. It seems unlikely that only class skills will be slotted on the hotbar so the passives might have more impact.
3) This should ensure even more variety between characters, but it really makes me wish we had a couple more hotbar slots. It will be really hard to choose!