TBH I find Estrogen's idea intriguing when applied to the divinity series. I'm imagining the DKS saga with a skill that will be available for warrior/priest/mage/ranger class whenever you have a given number of skills of certain levels in their tree. (e.g. 5 level 15 skills on the ranger tree unlocks an ultimate ranger ability etc.) It would be very interesting whenever you respec.
We can perceive it a glass half full or half empty. If the game was designed without heavy reliance on these overpowered, class specific/unique skill then everyone will still be happy and it becomes just a "bonus". However we can also see it as a limiting aspect that might discourage players from cross classing.
Also, there is something dangerous with the idea. If a developer does this on the per level basis then that would be very bad (like, the next skill requires x levels of previous skill). I wouldn't like that of course.
