1. The filter part
I can use extended tags, which I can edit with Mp3tag, and I can filter by those tags. For instance, I can add a tag "xrelax" to the mp3s that contains relaxing music, from that directory, then filter for the word xrelax. While "relax" might appear in a title of a song, xrelax will certainly not.
2. I can create different shortcuts, and each will start a different folder.
"C:\Program Files\Mp3tag\Mp3tag.exe" /fp:"C:\mp3"
So, I can create 3 different shortcuts, to 3 different mp3 folders, and perform additional filtering on each instance.
3. The interface part
Not only is very easy to navigate, but I it's made of modules, so I can close the modules I don't need. So, I can have one big filter box, and below it all the mp3s from that folder.
4. I can drag and drop from the interface to the mp3 player
This is very useful for me, because mp3s play with Winamp, but wav play with Audacity by default (because I usually want to edit the wavs). So, if I start Winamp, I can then drag files from the filtered list to Winamp, even if they are wav. This way, I can play wavs with Winamp, in an easy way.
5. I can create a playlist very easily
Combined with filter, I can create playlists just for what I want from the entire list.
Even more, that playlist is compatible even with old versions of Winamp
I just installed Mp3tag about 1 hour ago, and I already feel this is the very answer to all I need about manipulating mp3s fast and easy.