Pretty sure this is not a bug as it's working fine for me both in the main collection and in a playlist.
Have you enabled Preferences > Tags (2) > tag handling > enable direct editing of tags in the main panel?
Thanks for the quick reply and test. I knew that it might have been an issue with my skin, plugins, and so on, so I downloaded the latest version from the website and ran in through a sandbox before posting. After your "this is weird!" post, I did some additional digging and realized the source of the problem. There are two separate fields, "Year (xxxx)" and "Year". You most likely tested the Year one, since I just checked and it's editable everywhere. I was using using Year in my regular views and Year (xxxx) in the playlist view, and since the fild was sized too small to see the "(xxxx)" part of its description, I didn't realize that I was using two different fields...
Anyway, it appears that "Year (xxxx)" isn't editable at all, most likely by design. I'd still prefer it to be editable since I like to just use years and disregard the days and months, but that's obviously not true for everyone and the editing was likely disabled to prevent accidents. The solution for people like me is to change the non-editable field to the editable one, which I just did.
Thanks again for helping me figure it out as it was driving me nuts. You can just lock the thread and sorry about the confusion.