I would just like to pop in since I see people writing how it's all about horizontal progression, and how everyone has a chance to learn everything, and thus all have a shot at being anything with equal success.
This is not fully correct.
The main reason for this is the stat system (DEX, STR, CON,...). This will be fixed, and If you go with 20 INT, 20 CHA, 10 STR, 10 CON for example, you will never be able to fully excel as warrior even if you learned all 400 skills.
It is about horizontal progression, but at no point will everyone be equal, as everyone will always be limited by their initial stat distribution that can not be changed.
The point is if you are an archer, and you learn all the skills, you may become extremely versatile archer with an ability to adapt to many situations, but you will always (from min max perspective) only be an archer. You may wear a warrior gear, and use warrior skills, but you will always be at disadvantage from any warrior that chose warrior stats on start.
And this is the system that I actually like. It allows for great versatility, but still defines you as a class from min max perspective. (It still makes sense to have alts to play a different class, specially if you combine it with different race).
This is also good from PvP perspective, as people are allowed to adapt, but at no point everyone will be exactly the same.