Why am I predicting that my paladin in Skyrim won't work well? Resources. I need to divide my levels between health, magicka, and stamina.
The paladin would use melee attacks for most damage with strong healing to keep him alive as he wades into large groups of enemies. Melee attacks drain stamina. Healing uses magicka. Getting punched in the face uses health. That means I have to keep all three fairly high, or else I'll find myself lacking in damage, healing (health), or health, one of which contributes to enemies being alive, which means that indirectly, all three are factors in whether I die, which is usually a bad thing.
It's not really the base pool that concerns me. A bit extra or lacking here or there isn't my concern. Instead it is the regeneration that worries me. If I increase my magicka regen, that only affects half my resource pools (I'm dropping the concept of health as a resource). My melee is not helped. Or on the other side, more stamina regeneration does nothing for my healing. This means that I must balance my gear for two pools, always risking depleting one or the other too soon, with no ability to correct the problem.
Contrast this with a mage whose damage uses magicka and whose healing also uses magicka, so that the single stat is perfectly flexible. A shortage of damage can be made up for by risking lower health, while a lack of healing means a temporary reduction in damage.
It doesn't help that caster gear tends to be robes rather than heavy plate. Enchanting can help with this, so I'll leave it as a "player problem" rather than a "developer problem".
Allow players to pick which pools are consumed by resources.
Change my physical attacks so that they use magicka. We can pretend that I'm not swinging my arms and sword with muscles, but magic, making it a sort of short-range telekinesis. A wizard did it!
For casters, allow destruction spells to consume health. This creates an interesting scenario where their health rapidly fluctuates, with enemy damage and destruction spells hurting them, interrupted for the magicka-using heals to renew it. Or they use the magicka for alteration and conjuration, supplementing their offense even more, but at a terrible cost. This would be a great setup for a warlock.