It would be nice if you could easily (and quickly) select all of a "group", such as all files in an album or all tracks by a particular artist, or all files in a folder.
Use a filter. Filters currently require too much manual text entry to make this workable for quickly selecting subsets. Something you could do to improve the interface would be to add a "Filter by" selection in the context (right-click) menu. Use it by right-clicking any field in view and select "Filter by" and Mp3tag would automatically create a filter to select items with that field (or fields) having that value. For example, right-click on an album value in the file view and Mp3tag might create a filter like:
album IS "Yankee Hotel Foxtrot"
I realize that the columns in the file view can contain multiple fields, as well as scripting functions. If you like, you could limit the ability to simple columns displaying only a single field.