I think that there is a lot of great Arthas skins, same with the Tauren Chieftain, Blood Mage, Sorceress, Assasin and a lot of other popular/mainstream units/heroes. There is so many of them that I thitk they are a little bit overused, so I prefere to focus on other, not so popular skins...
If I submit i.e. Tauren Chieftain skin now, a lot of people will say:
"
Oh, whatever - just another TC skin. I've saw this a thousand times before, and the one or the other was even better than this one here, so - meh, whatever..."
And from the other site - if I submit a nice, detailed skin for the Temple of Tides, someone may say:
"
Hey, that's a really refreshing stuff, did anyone try do rework this thing before..? Maby I could use this in my new map..? I should PM my friend, he was looking for something like this just a few days ago..."
And so on. So if I had to choose, I'd definitely take the less popular request - just to make it popular