I've had some of my "test" MP3 files moved over to WMP12 and Windows 7 only to discover that the ID3 tag information doesn't show up in either. Haven't figured out the cause of this behavior yet either and it's driving me nuts. The tag information was there and showed up when PLAYING the files but not in the LIBRARY or EXPLORER views.
If you look at the files in EXPLORER, there is a tag editor down at the bottom - true, not nearly as complete as WMP11's Advanced Tag Editor function but there nonetheless. Also if you select the MP3 file and choose PROPERTIES, there is a rather extensive ability to edit the tag information, in fact I'm thinking WAY TOO MUCH ability and clearly designed to encompass all types of files. The only thing I could not find to edit was the album cover and of course MP3Tag does that nicely.
BTW, if your MP3 files show up in EXPLORER without any tag information showing and you know there should be some, then you've encountered the problem I was talking about earlier. I was able to fix the ID3 fields by using MP3Tag editor and 1-delete previous information then 2-extend tags. The result is to lose the old information (and the bug that keeps them from being seen!). After extending the tags I was able to reenter all the information using EXPLORER or MP3Tag editior. The bug was not in all of my MP3 files either, only some. I may have narrowed the bug down to files I created with Exact Audio Copy which I used to create the tags with in the first place - not a definite source yet though.