As to WHY we don't like mana systems, letís list the things that DnD wizards can do, and will do because of how the game mechanics function:
Manipulate other beings (Not any issue in Diablo, as there is no roleplay-involved character interaction, and it really wasnít much of any issue in the original NWN OC, to be honest; as I recall, ďCharm PersonĒ was a completely useless spell)
Raise their skill/attribute levels (Diablo didnít even have skills, as I recall.)
Cast massive area effect spells, such as lightning, withering plague and such
Have a familiar
Be nerdy (by and large the best one)
Letís list the things Diablo magi can do:
Cast Fire, lightning, and freezing spells
Cast them LOTS AND LOTS AND LOTS AND LOTS AND LOTS AND LOTS of times!!! WHOOPEEE!!
Every other mana system Iíve seen involved absolutely no manipulation of objects, self, or other beings, except perhaps to make any of the above a little more combat-ready. DnD mages have Fire, lightning and ACID spells at their disposal all at once, and none of them are obsolete. Actually, they also have the Shadow-Weave, and Psionics, but thatís a different story. At the higher levels of Diablo, even if you TRIED to balance out your skills, your character will be a miserably weak one. Thus, a mana system doesnít allow strategy.
It was kind of the same thing with KotOR; you had to upgrade spells, and there were only a bare thimbleful of spells that Iíd ever use.
Force Lighting, Force Affliction, MAYBE Force Drain, and MAYBE Force Choke. There wasnít any particular strategic reason whatsoever to decide on Force Lightning over Force choke or the other way around, and either of those spells at higher levels with lots of Mana at your disposal would totally invalidate any need for Force affliction.
Lightside, you have Force Armor, Force Speed, Force Push, Stasis. Destroy Droid was pretty useful for fighting droids, but thatís the extent of changing anything. Stasis was kind of useless in light of being able to hammer on the Force Wave button, and I remember Force speed made you pretty unstoppable, and Force Armor was mostly for looks. When you can destroy all your enemies in one turn with the single press every five minutes of Force Speed, itís wasted effort to try much of anything else. It was fun to hack apart enemies after freezing them with stasis field, but the novelty wore off when you realized it would work every time, and that you didnít even have to use a quarter of your mana. At the height of frenzy, I might have used three, maybe even four spells. But that was kind of rare, and even on the highest difficulty that was just me over-shooting and trying to have fun by doing something other than pressing the same button. The button mashing wasnít even as good as Diablo IIís, although the combat far surpassed everything in looks and style and feel, I give it that much.
I remember some combats in BG II that I would use TEN DIFFERENT SPELLS IN ONE COMBAT, of necessity to my strategic options. I mean, wow, weíre talking variety here.
Where did all that variety go?
The spell-slot system allows the maintenance of multiple tiers of spell, rated on how powerful they are (in some obscure way, I donít know; itís a decent estimation, at least), to be cast in conjunction with eachother. Having these tiers bypasses the problem of running out of mana, and thus any bother to make excessive use of these potions and spam your highest level spells. Unlike a mana system that doesnít recharge or recharges slowly during combat, once you cast your higher level spells, you still have lower level spells to cast. If you choose unwisely, though (as I see my characters casting Finger of Death on the ferocious goblins biting at my ankles), you donít have those spells when you need them. Lower level spells still have their specialized uses at higher levels, where they havenít in any Mana-system Iíve seen. Non-combat spells may be cast, and they will be cast with equal exertion as a combat spell. It makes the player balance out the spells in their slots to befit situations that may arise, giving meaning to spells that would be neglected. In compensation, the spells have many more levels of power and variation than any other mana system (that I've seen), making the mage an extremely powerful specialist in combat - not a combat mage. They can do more things than any other class in the game, they are more versatile and more capable of instantaneous mass destruction, so is it really so bad that they can't manage the right end of a sword?
This isnít a good thing for a combat-centric RPG, butís a great thing for roleplaying in general, because it validates spell-casting as a roleplaying tool - in a sense, I suppose, it expands the combat field beyond the combat and into things less direct. Still, there is no ďCombatĒ in ďRole-Playing GameĒ.
My point? Thereís yet to be a good mana system (that I know of) that didnít preclude a combat-centric action cRPG, and had nothing to do with character development save that the avatar on your screen developed into a stronger being through the course of the game.
Is that answer not good enough for you?