The ability to allow the user to chose what skills he/she can chose from on level up using skill trees would be amazing. Choosing what levels the character will be able to choose a skill from, and on that level give him/her a choice to choose... let's say, you just got level 10 and now you can choose from Heal, Fire 2, Ice 2, Water 2. This way you can make your character exactly how you want him/her to be, and not how the game designates you to be. It allows customizability, and creativity with the user.