A good example of a villain that is not evil for evil's sake is Jacqueline Carey's Melisande. Melisande love(d/s) the two main characters, and what she does she does not because she wants to destroy a nation, but because she feels it's for the good of the nation. (Myself, I don't see Voldemort as a good villain--Snape was much better as a gray character than Voldemort as a villain. Voldemort was pretty much evil-because-he's-evil.)
Personally, I like villains that are 3D. Which, I think, is what a lot of readers want, lol. I don't mind if a villain is all-evil...some people ARE just psycopaths...but I like them to be interesting. For example, Terry Goodkind's Darken Rahl was a character I liked. Too bad he was only used for one book; the replacement bad guy Jagang wasn't nearly as cool.
My own issue with creating villains is that I sympathize with them too much, and they become anti-heroes, lol.
I think a way to make them not as powerful is to carefully consider what powers they have, and their wants and needs. For example, say your villain is...oh...an avid admirer of art. Having that villain run around destroying everything will ultimately squelch innovation and creation, leading to a duller future for the villain. If you make your villain intelligent enough to know this, he/she will rein in their own power just because they don't want to be lord/lady of a pile of burning rocks and barbarians that grunt and think stick figures in the dirt are art. Or maybe your villain really loves their mummy and daddy or something...social pressures work on villains too. "Don't you dare burn that village to the ground, Junior! I WILL disown you!" (of course, more subtle)
So I guess the trick is, give them human motivations, wants, and desires, and the intelligence to know fire-bombing everything isn't going to be productive to those needs. Give them a good reason not to squish the hero like a bug. (key word being "good" reason; there's a lot of sucky ones out there!)
A thing that I consider is that even though a marathon olympic runner can do a whatever-yard run in X seconds flat, they can't run three races at one time. Similarly, even the best mage in the world can't coherantly multitask and track X number of enemies all at once. Or maybe the best mage in the world CAN do that, maybe the best mages are defined not by how much power they can channel but how well they can multi-task, but they suck at battlefield tactics so if they can be lured out of their hold onto a battlefield, the hero's best general or something can get 'em physically. Or something. A wise leader (even an evil leader) knows their strengths and weaknesses, and looks to shore the weaknesses up. But they *will* have weaknesses, which *doesn't* negate the fact that they have strengths too. It's up to you as an author to seriously consider what their weaknesses are, and how the hero discovers them/stumbles upon them/etc so your hero can defeat the villain. (And to make sure you don't fall into the lock-of-hero's-love-interest's-hair-ground-up-and-fed-to-villain-in-a-poison or something silly like that trap that provides a pseudo-plot to abduct the hero's love interest--those sorts of weaknesses are weak. lol)
Here's hoping I make sense...
