I don't actually know if there are DW equivalents of the rant comms, but there *is* fandom_rant at journalfen, which has 1500 members and gets a fair number of comments, if you're willing to spread yourself around even more sites...

And, yeah, the etiquette thing is part of what makes me hesitate about starting a new little-details type community (and about really advertising the DW tgs community, too) - even if the mods on LJ have shown no sign of wanting to leave, it still feels vaguely like I'd be trying to take something away from them. :/