Don't make me poll. For me, this is the most annoying aspect about the current generation of art websites. I'd like to see new gryphon (griffon, griffin...) art. Currently, this means I have to explicitly search for it. It'd be awesome if a site informed me when there's new gryphon art to be viewed. Polling for it is stupid.
Don't lump all images together. There are some artists I'm more interested in seeing updates from than others. When there are a lot of new images and they all get lumped in together it's easy to miss the interesting ones mixed in with the rest of the crap.
Don't make thumbnails so tiny. Most of us aren't on dialup anymore. It makes it hard to pick out the interesting pics from the rubbish.
I don't give a shit about stories. Most fiction on the internet is crap. I'm not interested. Same goes for poetry and music. Go away.
Don't just give me a list of titles. DevArt used to do this if you weren't a paid member. FA still does this with journal posts. I'd much rather have a LiveJournal style list of blog entries. Not providing thumbnails for images is intensely aggravating.
Tags are FTW. They make searches so much easier. Tags should have aliases (different spellings, related terms). Anyone should be able to tag an image, and people should vote on tags to give them greater weight.
Site banners should be unobtrusive. I frequently browse using my netbook, and large banners take up valuable pixels.
Site navigation. The main site page should be my personalised feed (eg. Twitter). I should be able to jump to my profile/settings easily from any page. I should be able to find any artist's profile page without difficulty. The navigation through galleries should be clear and logical (eg. don't show a "next 36 images" button if there's only 5 images in the search results!).
The site must be fast and responsive. Sometimes I click on "full view" of an image and nothing happens. Did I actually click on it? Is it still loading? I don't know!
Ads unless you give us monies (or you run an adblocker). This is offensive. I know sites need to recoup their running costs, but surely there's a better way. Value-added content for paid subscribers?
I'm sure there are plenty more problems.