Submission#-Artist-Title.extension would work best I'd say, I occasionaly like sorting my downloaded art by artists.

Also, right now saving art can produce three different files:

Clicking the download button gives you a string of characters

Clicking Save As on the submission page yields 2500x1500.png

and Clicking Save As while viewing the submission just gives you index.png.

Would be nice if all three actions gave the same result.

I second this. Getting to have the name I want is pretty much a deciding factor on whether or not to make an account \o/