There is an innate uniqueness to the classes, some of which you have stated. The trouble with "depth" is that it comes at a cost. Paladins have less HP than Warriors but their mitigation is far superior to make up for this. Warriors have nearly double the HP (depending on gear and buffs) but mitigation is lackluster and they hit harder.

I use those as examples and I could delve into Dps, but there is no need. Monks are different than Dragoons, Black Mages than Summoners. You as a player make them unique too. And while options to do so are limited you can play outside the box