I was keeping a list at a previous company:
- default text size is too small, text block widths are often not constrained enough getting the "slashdot unreadability effect"
- 0.5s+ lag when clicking any textbox
- 1s+ lag when clicking any dropdown
- 1s+ lag delay when grabbing and trying to drag-and-drop to reorder items in the backlog
- 2s delay when clicking in an issue in the backlog before the loading starts (add about 2s more for the loading itself)
- "Link issue" using a hyperlink icon. The whole idea of using the word "link" to talk about related issues without spending some time to think whether that's really confusing with hyperlinking
- Error messages about what "might" be wrong with things rather than what is actually wrong: "We couldn't save your comment, it might be empty or have invalid formatting". Well which one is it? Did you really have trouble deciding whether my comment is empty or not? Be specific and tell me what exactly is wrong, or do you want me to binary-search / bisect myself?
I think most of the issues can be fixed by completely rewriting the frontend.