Easy way to slowly introduce game mechanics and keep players interested for what is coming next.
Talent trees make the player feel a gradual increase of power.
Players should have to make choices and feel the consequences of said choices.
A character shouldn't be able to unlock and excel at everything.
Stat increases in talent trees are boring, skills are exciting.
Good talent trees should interlink.
Offer advantages and disadvantages. Example: A character that chooses to go the archer route should be strong at ranged combat yet weak close quarters.
Talents unlocked in talent trees should not be fillers or something that is already expected at the start of the game.