I like to use MP3Tag to create playlists for my favorite audio application, Spider Player. I have even recommended your application in the Spider Player forum. However, your application lacks one thing - the ability to start out with a blank playlist and add individual files to it. Any chance this will be added in the (near) future? 
You can select all files and then use DEL key to remove them from the list.
Then you have a blank playlist.
I take it then, that deleting items from the list view doesn't delete them from the computer. Good. However, I still can't add individual files - I have to add entire directories. I suppose I could open windows explorer and drag the files into the list, but I can't see all the tag information to make a decision as to what goes in the list and what doesn't.
On a different subject, I've noticed that, when changing directories, double clicking on the directory in the "Browse for Folder" window does not select the directory - could you add that feature? It may also make things easier if multiple directories could be selected from that window.
Yes, you can add individual files via drag'n'drop,
This is a Windows dialog and I can't change its behaviour in this regard.
Yes, you can add individual files via drag'n'drop,
Very useful it is too, but note Florian that this feature is virtually a secret to most users. It is undocumented in Help and unavailable on the menus. I suggest the menus need updating to support these recently added filelist operation. File | New and File | Add file... (track or playlist) for starters.
Well, that's good but, as I mentioned, filenames are not always sufficient information to decide whether or not to include a song in a playlist. When creating playlists based on rating for example, or creating playlists from genre information.
You can enable display of ID3 information within Windows Explorer or use a third-party application like AudioShell for this.
You can enable display of ID3 information within Windows Explorer
WMA too... but not APE to my kwowledge.
My ideal solution for playlist composing would be mutliple Mp3tag instances. Then one could drag tracks from a library instance to a playlist instance.
This might be a good quick and dirty fix - but my point wasn't so much what a good workaround for this problem would be. I'm grateful for the ideas and pointers, but I think that MP3Tag should be able to do this without workarounds or dependence on third party applications - and it certainly should NOT depend on WMA! If I liked WMA, why would I use MP3Tag? I could edit tags and create playlists from WMA and toss MP3Tag in the trash! 
Actually that would be a slow and clean fix!
I think that MP3Tag should be able to do this without workarounds
or dependence on third party applications
Well, Mp3tag already can do both the things you asked for: "start out with a blank playlist" (using Options | General | Favourite directory) and "add individual files to it" (drag'n'drop).
- and it certainly should NOT depend on WMA!
No solution suggested in this thread has depended on WMA.
If I liked WMA, why would I use MP3Tag?
Because Mp3tag is the best tagger for WMA! 
You suggested WMA for display of ID3 info. It depends on WMA because Explorer does not show tag info, such as rating. If you are aware of a technique to display rating info in Explorer, I would like to know how - I can't find a column for it.
I like MP3Tag - my only purpose here is to suggest some ideas that would make your application a little more useful . . . at least to myself. If you think it's a poor suggestion, that's fine - it's your application. I'll just be on the lookout for a more flexible application with programmers who are a little more attentive to user suggestions, that's all. 
You suggested WMA for display of ID3 info.
Er, no. WMA is a format - it doesn't display anything. I suggested that just as ID3 is displayed by Explorer, WMA is too.
If you are aware of a technique to display rating info in Explorer, I would like to know how
Sorry, no.
Sorry - my mistake.