Skill points from *more difficult* quests?
Though, most offensive spells currently are just "damage", perhaps modified with terrain as well. For example, I could dump points in Shocking Grasp and level quite well in Jurassic Ocean, but then get 0.16 multipliers at times in Little Italy. I'm not sure if I'm generalizing this, but choosing spells then would just mean choosing the zones you would want to level in, if looked at in the levelling point of view.
Also, some of the T3/T4 offensive spells are not as good as the T1 ones. For example, I believe Demonshreak changes its multiplier based on the alignment of players, so you have a 1/3rd chance (slightly less, since players/mobs aren't equally aligned distributed) of doing major damage. Unfortunately, harm can achieve a multiplier of 2.60 (or 2.70), and that's a T1 spell. Mana blast, Electric blast are also spells that have potential, but are outclassed by lower tier spells.
Not sure if such a classless system would then lead to players (optimally) choosing the few offensive spells which will give them the most bang for their buck, unless these spells were revamped.
*Disclaimer: This is what I remember from a few months back, so the spells could very well be different.