Since I think most of us want the best of both diablos bundled into Diablo3 I would certainly like to Diablo2's skill system to be back but I would welcome a fourth tab with "acquired skills" where you could once again learn skills (or maybe even passives) from books. Just like you could in Diablo 1.
I dont have too much insight on how the skills would work, but I would suggest something like this:
*Acquired skills lvl up everytime you read a book, just like Diablo1.
*But you could lvl up any skill learned from books until lv 5.(which would be a big selection of skills, like Diablo1), so you could identify which skills you would find useful.
*You could choose 10 skills to be specialized. Those specialized ones could be lvled past lv 5 until lv 20.
*Requirements to lvl up those skills would include "Magic" (or energy, whatever it will be called) for spells or "Strenght" "Dexterity" for passive skills, player lvl, maybe even some achievements like "Defeat Diablo on Hell difficult" or "Find The *Unique*", and so on and on.
Whatever, I just would love to see a mix of Diablo1's book skills and Diablo2's character skills. I like how Diablo2's has more specific skills on the classes but I would like to see the books return as well, I am sure if done right It could be a lot of fun because it would diversify the skills choices a lot.
And of course that those skills would be mostly "support skills" (like heal), it wouldnt be quite possible to built your character based on those skills (the class skills have that function) but it would be that trumph card or extra little trick that could help you in times of need.