(1) Since it's difficult to find people running 800x600 resolution anymore, why not standardize the next major version of BRU on 1024x768 resolution?
(2) Don't implement tabs unless: you want to hide the RegEx section behind the Replace section in one tab only. That would save space if you need more, and though I would not know, I would guess the vast number of renaming actions occur simply using the replace boxes over the RegEx feature.
(3) Finally, remove the "Subscribe to mailing list" tip from the Tip of the Day box and replace with a note about the new forum. You're right — the forum is far more efficient.