Well, in general I agree with Chris. It depends a lot on the audience that you're writing for, your genre, your niche, and a whole heck of a lot of other factors.
That said, here are my personal opinions:
It depends, for me, on what you mean by lackluster. If we're talking about not being particularly good or bad, than yeah, I think story can make up for it. Think of Stephan King: I'm not even close to a fan of his writing, and yet he's clearly a huge commercial success. He preaches the KISS method of writing, relying on his storylines to carry the book along. In terms of fans and finances, it works wonderfully for him.
It's mostly true for how I read, as well. Provided the prose is just basic (not bad, per se), than the storyline is what will win me over. Give me a character to care about, a plot that I can't set down, and/or some hot sexin' and I'll stay with a serial to the end. But give me three pages of great prose with no plot, and you'll lose me quicker than bad directions from Mapquest. There's just nothing for me to invest in, nothing to care about -- I can't daydream about what spotless prose will eat for lunch, or wonder what it would be like to be stuck on an island with spotless prose. For me, even the best prose is bland without a strong concept behind it.
The characters and plot keep me coming back, and that's really what I rave about in my reviews here. I read web fiction for the niche, not the prose. IMHO there are plenty of well-written published authors that I could read for great prose, and even more amateur writers that have their work available for free. We're talking hundreds of thousands, provided I'm not picky about content/want non-niche content.
Unfortunately, there are significantly less stories that I can find if I wanted, say, a lesbian, erotic, urban fantasy story. I'm more likely to forgive bad prose/web design/updating schedules if a writer gives me something I can't find elsewhere.
Standard YMMV Clause: People who have different reasons for reading will have different desires, of course.