There's nothing like a really well-drawn pencil comic, from an artist who understands what can be done with pencil shading. Which reminds me of that urban fantasy webcomic that disappeared a while ago, something-bookstore, IIRC.
As for things that annoy me....
Poser. Or other CGI comics. Sorry, but even aside from the Uncanny Valley aspects, CGI figures are way to static and stiff appearing to work for me. Especially with the creepy expressions.
"Science Fiction" webcomics where the artist's only notion of space science comes from watching Star Wars and Star Trek. Unfortunately there's a popular webcomic that combines both the above annoyances, so even though the writer seems to be telling a good story with interesting characters, I'm avoiding it.
Artists with poor grasp of perspective, anatomy, or action. Whenever I look at a character and wonder, "Has that artist never seen an arm before?" it's annoying. Note: this doesn't include comics where the artist obviously knows what he's doing, and is doing a -particular style for effect.
Webcomics with no sympathetic characters. Especially ones where I want to personally strangle all of the characters, stomp on the remains, and light them on fire, whether for being aggressively stupid, or simply being utter jerks with no redeeming values. Anders Doesn't Love Anybody, I'm looking at you.
The above, especially if they are supposed to be autobiographical comics. Man, if you're such a loser, why do you think I want to know about your life?
Imitation is not the sincerest form of flattery- even if it's accidental. For example, if it's a neo-steampunk comic featuring a blond, long-haired, glasses-wearing girl inventor with an affinity for music and her name ISN'T Agatha Clay, then well....
Not deal breakers, but:
This is a bit hard to explain, but- black and white work that was designed for color, especially in the style of mainstream comics.
Crazy for the sake of being crazy characters- translates into "crazy for the sake of being annoying".
Websites where I can't navigate the archive by url. That is, if I am on comic 273, and I haven't read this comic since page 151, I want the past pages to have a page number in the url so I can just type in the number of the comic I want.
Sex comedy comics that show sex scenes, but for some reason cover up all the "naughty bits". though I guess I just consider this more odd than annoying.
Webcomics where the main female character is the only sensible one who acts as "straight women" to the male leads, as well as being somewhat hapless.
Likewise, if she's the only competent member of the cast.
Especially so if that woman is the only cast member to appear on the front page of The Webcomic List, with cleavage exposed. Especially because that tactic seems to work, damn it.