I've been using BRU for a while and I must say it's just awesome. I tried many other renaming programs over the years, but for me the winner is BRU.
Ok, enough of that

My first suggestion is to add the ability to change the font (typeface, style and size) for the tree and file list views. Why? It's not just a comestic change. I'd find *very* useful to use a fixed width/monospace types such as Source Code Pro, Droid Sans Mono or similar. Proportional typefaces are nice but in this context they're not the best option. A monospace typefaces would make very easy to check visually if certain parts of the filename are in the same char positions. Due to their nature, I think monospace types are best suited for this kind of application. And if implemented, it could lead to other potentially great functionalities like multiline name edition which would be unpractical with proportional typefaces.
Regards, Juan.