This is a list of things I can't stand in digital and online comics in no particular order.
1) Unskilled use of dodge/burn to do shading. It never looks good, it just looks as though your characters are made of shiny plastic.
2) Excessive copy/paste. Now and again it's alright, but when your comic strip consists of the same frame copied/pasted 5 times, it gets a bit boring to look at.
3) Bad scanning. Clean up your scans people. If you're going to use hand-drawn art, make sure it looks nice. If you're using scanned lineart and digital colouring, for the love of god learn how to colour underneath your lines and don't just use the fill tool.