What's your favorite boss mechanic?

It seems to me that it's got to be tough for MMO developers to constantly come up with new boss fight mechanics. I mean, consider how many games and bosses have been made by now, and yet players still expect something they've never seen before. That's got to be daunting.

I'm somewhat fond of bosses that transform into different modes during a fight. That's especially nice if it's a long fight, since it's a good visual indicator of how far we've come. It keeps things spicy, too.

I'd love to hear about your favorite boss fight mechanic today. Let's go beyond the red circles of doom and dig deep, people!

TH