Better notification system, for sure! Tapas wants us to use our wall for non-comic posts but c'mon... no one reads those things. This might be in part because wall notifications are lumped in with comic feedback, and because wall comments can't be replied to individually, so everyone gets notified of every comment. Users should also be able to opt in or out of comment threads or post types.
A block/report user function! Fortunately I haven't needed to use this yet. But it seems like a pretty basic function for a website of Tapas' size.
Trending/Popular Comics by Genre- I'd like if the front page could feature popular/trending comics from each genre, rather than having them all lumped together. It would help bring more diversity to the front page. As it is now, the front page is usually dominated by romance and slice-of-life comics. Webtoons sort of does this with its front page "Genre" bar.
I also love @GoldenPlume's idea of suggested comics based on what's in your library! This would help retain readership on the site as users wouldn't have to spend as much time digging to find the content they want.