Initially, art for shoujo will always be first, plot second. Actually if we were considering genre/subgenres too, I'd have to put genre before plot, lol.
To be fair, if I pick up something with nice art and it puts me to sleep/ or it seems pointless, I'll drop it. However, most mangaka's with nice art are the ones with the experience (usually) and with all that said experience, more likely than not, they can create a decent plot to keep me engaged.
If a series has a great plot with meh-ish art but shows signs of improvement in the art department later, I'll give it a go too. These are usually the old, longer shoujo series.
Although this is irrelevant, I'd say if a series has great art and an eye catching plot, I'd grab it if it's not usually the genre (romance/shoujo) I read. I'd admit though, if I see a series tagged with ecchi, yaoi, yuri, tragedy that is an autokill for me, lol, I won't bother with either the plot or art.
I'm not a big fan of older shoujo art; I really actually hate it. But, gosh, the older shoujo series have the best plots. The art even starts to grow on me and improves over the many volumes. Like Pharaoh no Haka, Red River, Tokyo Crazy Paradise, Ingenuo although I haven't finished reading all yet, I do like them very much.
Other cases like Bride of the Water God, which is gorgeous... well, plot wise the way it's going now makes me want to slam my head on the desk, repeatedly.
For art probably what I'm most picky about are eye. I hate huge, starry/bubbly eyes with the tops of white showing and unnatural, measured lashes and stick eyebrows. It just makes me grimace in pain. And this coupled with bad plot is a killer.
I know I'm biased and picky as hell.