Its not just webfiction. Theodore sturgeon, ome of the greatest sff / speculative fiction authors ever, once said, about novels, 90 percent of everything is crap. Yes, most of the webfiction sucks. But there are people that won't even notice the errors, and enjoy what they read. Look at :shudder: rebecca black.
That said, there are gems out there. And yea, they are mostly people who spend the time, and who get an editor, either a few fans, or an official editor. I work as fan editor (dedicated fan who is the first to read new posts and suggest changes) for a couple of writers, one of who actually trusted me with admin access to their site to change things for them. And I'm the actual editor for a few people, two of whom are hosted or will soon be hosted by me on my website. And I wish I had a good editor to look over my own work, because it is NOT easy to be your own editor, in the slightest.
So I think you are approaching things from the wrong angle. Stop thinking, "my gawd, this is shit". Start thinking, "well they have good characters, a good plot. I like the story... So... what can I do to HELP them make it better?" Most of the authors out there, point out the flaws. Point out the typos. They will be genuinely grateful. For them, the story is where the quality lies. But the furniture helps.