This one's easy. Lyrics.
Don't get me wrong, I think good "flow" is important (whatever definition we're going with here) but not entirely essential. I will always prefer a song that has great lyrics over a song that has good music/flow and average lyrics. For me, music is about conveying ideas. Not all of these ideas can be channelled through an instrument. What's the point in creating great music if you're just going to plonk some juvenile poetry on top of it? Freedom has a scent like the top of a newborn baby's head... Might as well just write instrumentals if you're going to churn out crap like that. Bad lyrics just distract me when I'm listening to music.
That said, there are A FEW bands that I like purely for the sound of their music. These tend to be poppier bands/artists (The Beatles, Red Hot Chili Peppers, etc). Not to mention bands whose lead singers have vocals that sometimes function more as instruments (Sigur Rós, Radiohead, etc). Still, I'd rather listen to good lyrics that centre in on topics that are important to me.