It's a diverse crowd. You get the critics you ask for, more or less, and it depends on the kind of story you're writing; people will read the stories they like, so there's self-selection there. You can for example compare the discussion I got here with the more typically shallow analysis here.

It's absolutely focused on fanfiction, but that hasn't stopped people from posting originals, usually to great success.

Mostly, the keys to success are: * State up-front that you'd appreciate real C&C, otherwise it's likely to turn into a bit of an echo chamber. Depending on the kind of story; quests naturally cause more discussion. (But also have a smaller base audience.) * Write reasonably well, about interesting subjects. The bar is low, but it's there, and original fiction has a higher bar. * Sticking a link in your signature and posting in other threads always helps. If you like reading, you should be able to find something to read there, somewhere, so that'll go naturally.

On that note, why not write fanfiction? It's fun. :P