I can't actually watch/favourite, it gives me the "need to be logged in" error, but then redirects me to a 404. This forum doesn't have watch as a guest, but yours does, not sure if it's an ACP setting, but I don't recall seeing it. I've got a few bugs I've spotted whilst using my iPad that I'm going to put onto GitHub, but I'll do some more testing later into guest permissions.
Edit:sorry, when I said replying and voting are fine, I meant they don't redirect, I can't actually reply or vote. I need sleep.