I was just looking at the Bad Webcomics Wiki, where all the haters come to talk about comics they hate. (And yet, have mysteriously read from beginning to end.) I don't agree with all their criteria (bad art does not a bad comic make, right?) and some of the reviews are pretty spiteful. However funny. But they're right about one thing--you can improve your work by noticing others' mistakes. I'm scarcely an authority on the matter, but I've started reading a lot of online comics lately and here's some errors that pop up again and again:

Ugly website. There should be art on the front page, so visitors can decide if they like it without having to click around. Navigation should be clear and logical. And if you have a gazillion pages in the archive, you get bonus points for making your major story arcs and/or greatest hits easily accessible to new readers.

Typos/poor graphics formatting/general sloppiness. Get someone else to look at your work before you post it. They might see something that you missed.

Missed updates. True, most of us artists have day jobs and busy lives preventing us from drawing as much as we'd like to. But that's all the more reason to choose a less frequent update schedule (or none at all) instead of committing to daily or weekly updates when it will only cause you stress and piss off your readers when life gets crazy.

Self-referential humor. For the love of God, don't have every other joke be "Hey look, we're in a webcomic." And to borrow a phrase from Tanya from The Webcomic Beacon, when you make a webcomic about making a webcomic, you're really making a webcomic about...nothing. Go ahead and peek through that fourth wall every now and then. Your readers will indulge you. But to do it all the time isn't clever or original--it's lazy and solipsistic.

Veering off-topic. How many times have you seen a webcomic description like this: "My strip is about the crazy randomness that is my life and all the different stuff I like." And how many times do you click through? Readers want to know what's in it for them--what kind of content they can expect--and they like it to stay relevant. If your strip is about say, gaming, and you ignore gaming and get all political for three months because it happens to be an election year, you're going to lose people.