Things that are/were irritating to me the most:

• Polish major platform doesn't allow multiple reads of the same book. I want to track the number of books I read each year and this was a problem if I reread something: I had to decide when to mark it or mark other edition to count it.
• Goodread's mess with book editions. There is no filtering on the edition list, so something like "Son Kichot" has thousands of editions and good luck finding yours.
• Goodread's inability to properly mark releases that are split in parts. So the LoTR: Fellowship is T1 of the series. And someone could argue that The Hobbit is T0 (there could be also something like short story between them - T0.5). That's okay. But now someone is releasing the Fellowship in two books. And the system doesn't work anymore as usually these are added as different works of Tolkien. It's half policy half design problem, but it makes checking the author's bibliography painful.