I've heard the word "but" referred to as "The Great Canceller." Linguistically, it ususally cancels out EVERYTHING that came before it, the subconcious effect is to say "forget all that, here's the real point."
We've all seen it: "You're doing a really great job, but...", "That's a really good point, but...".
Once someone pointed the phenomenon out to me years ago, it's like a red light flashing every time "but" pops up! If Bush could point it out clearly and with good humor, it would totally change every person's experience of listening to Kerry, because he does it constantly. Once you're conscious of it, it's very hard to miss.
Absolutely true. I will be paying close attention to this in tomorrow's debate.