Motivation is probably the most important.

With good motivation ideas can be developed over time. You can have all the best tools, but without motivation they won't do you a bit of good because skill can't be bought with money. Skill is bought with practice, and to practice takes lots of time and (you guessed it) motivation.

MOOOOOOTIIIIVAAAAATION!

MOTIVATION. If you don't even have the will, what's the point? D;

motivation :3

Motivation rules them all! \o/
Really, without it, it really doesn't matter if you have ideas, skills nor tools.

Motivation.
Don't be lazy~ *stares pointedly at self.. how is that even possible, anyways*

Motivation brings everything else. Skills, ideas and tools don't matter if you can't propel yourself forward to actually act on them.

An artist needs to have guts and gusto!

I say motivation, skill can ve developed, ideas too, with time the mind starts to work mor efiently in that area, tools can be obtaines with time too, but motivation has to be there nad be natural can't be learned, or developed

I really wanted to say motivation, but I think that ties in with skill. If you don't know how to use your tools and the picture you end up with doesn't turn out to be as good as you planned it or it takes a long time to do because it's so hard, then that hurts your self confidence a bit and makes it hard for you to want to start again.
I honestly say this from somewhat of a personal experience. Say I'm having trouble during the sketching phase or some other phase with like effects. I'll get really frustrated, take a nap, and start over again. I don't like to feel like that.

Motivation.

You may have the skill, but don't feel like usin' it.
You may have the tools, but prefer to see 'em collecting dust.
You may have the ideas... but if you don't put 'em to use, they'll stay that way.