Same as for a print journal, really -- I look, first and foremost, at the quality of the work they publish. I also consider how "professional" the journal looks, and whether they make any effort to promote themselves so that, if published there, I could know that my work would be read by someone other than the other authors in that issue and their mothers!
Also worthy of consideration is how long they've been around and who, if anyone, sponsors them -- an online journal that's sponsored by a university and has been around for a few years is way more likely to still be around in a year or two. The presence of an ISSN is another indication of a journal whose editors take it seriously and intend to keep it around for a while.
Good luck with your venture -- be sure to let us know about it.