Without an idea, the skill is almost (but not quite) irrelevant. This is more true in writing than art, where the sheer colorful exuberance of a picture can be enjoyed despite the lack of "real" content.
for me its a mix, i've seen beautifully well done paintings from the Italian Renaissance that have completely lost me because the where fillled to the brim with symbolism and messages all about how the church is awesome and pagans are bad to the point in which the museum could have installed a speaker next to it play a loop of some guy shouting "BURN THE HERETICS!" and it would have had the same effect.
and on the other side of the spectrum i've seen things like dwarf fortress who have almost no astetic value to the point in which i cant even begin to figure out whats going on and get to the pretty awesome concept that i keep hearing people prattle on about.
that all aside there's also the style to think of, a lot of flaws in concept or art can be excused if its being done in the right style, i mean in a cartoon-y piece you can excuse some hiccups in the art and definitely in the anatomy here and there as long as the idea behind it is good, but in a more realistic piece a woman's nose being tilted too far to the left compared to the position of her head might be enough to completely suck you out of it and make you lose track of the message.
...i duno, i guess what im getting at is its all super situational and hard to judge as one would think when dealing with something that subjective
Gotta have a good mix of both, in my opinion, however, I'd lean towards "The Idea" a tad more than the level of skill. If it was not for "the idea" many art movements simply would not have existed. Art is often used to send a message (at least it used to be), and if you have no message to send then it just becomes a bunch of pretty colors with little meaning. Of course, a good amount people like pretty colors and stuff of that nature anyway, so there is still value to be had there.