1. Twenty-something loser guys who have lame jobs and lamer love lives, unless there is something more to them. (Daddy issues don't count.) Seriously, this kind of guy makes up a good chunk of the protagonists in submissions. Is it a lack of diversity in artists that is causing a lack of diversity in characters? Perhaps. But I think it might just be artists' narrowness of scope. That's a bigger issue than I really want to get into now, though.
2. Comparisons of your work with TV shows or movies in the cover letter. I have heard the "Big Two" encourage this kind of self-loathing, but I say love the media you're using, dammit.
3. Storybooks in rhymed verse. (Especially true if the verse does not have any kind of consistent pattern or meter and you expect us to print it in color.) This is a hard sell for us, as well as for you. Think for a moment. How many rhymed-verse storybooks have we published that weren't by artists with whom we worked for some time and who have an established audience? How many of them were in color? When was the last one published? (To the best of my knowledge: Very few, if any. None. 1999; unless you count a back-up story in a prose storybook--then it's 2002.)
4. An incomplete synopsis. You have to tell us how your story ends. A coy question-ending to a synopsis like, "Will Trixie discover the dark secret behind Troy's strange behavior?" does you no favors. And it makes us suspect that you don't know the ending of your story. (On the other hand, don't go crazy; you don't need to include every little detail -- try to keep the synopsis to two pages, single-spaced.)
Another tip, but it's not on the list since it's not a particular prejudice of mine, just a reflection of the current state of the market: Don't tell us about "issues." We're just not very interested in series right now. If you take a look at what we are currently publishing, there are only two creator-owned comics still being published as a series--Nightmares and Fairy Tales (which is ending at issue #23) and Rex Libris. [EDIT: I should clarify: These are the two titles that we are still publishing as regularly scheduled series. We still are publishing intermittently-scheduled comics by some of our established creators, like Dork and Lenore. The market is just in such that it actually seems to be easier to get people to try new artists' work in graphic novel format than in floppy format. It's interesting. Perhaps because it involves just a one-time purchase and the people buying aren't "visit the comic book store every week" sorts of people. Perhaps it's something else. But, right, just to clarify: I mean to say that we are not really considering new series. ] The other series are Disney-licensed comics. You should think of projects in terms of graphic novels.
All-in-all, there are more things that don't trigger negativity in me than do. I'm often impressed with people's creativity, even if their project is not something we decide to publish. SLG welcomes submissions -- most of our new projects come from the slush pile, and we try to give constructive feedback. Read the submission guidelines at our website to learn more.