Funnily enough... some of the things being requested here are things Tapastic used to do but no longer do...
Anyway, aside from things that have already been brought up.
1) A better rating system would be nice (not sure if this was requested, I read really fast through so may have missed some stuff thinking it's a re-peat since there's a lot of things being repeated), mostly so I know if the NSFW is for violence, nudity or sexual content. Right now I pretty much always assume it's porn since that's the most common thing to be behind those walls.
2) A way to post extra content WITHOUT IT BEING ANOTHER NOT-COMIC SERIES. Or even a way to label it as NOT a comic. (Those gallery or 'extras' comics is what I'm talking about)
3) The ability to add translations WITHOUT making it into another comic. I know this is another feature we used to have (though it was limited to only 1 additional language) that was removed, but the cluttering up of multiple versions of the same comic just doesn't look nice.