Maybe these two things are related, but having just given my notice at a big corporate software company my experience is a bit different; It wasn't the bureaucracy that drove me away it was the mediocrity.

Day by day, I saw us doing things to account for the fact that we had a lot of mediocre developers working there. Worse yet, I felt myself sliding into mediocrity as well.

Perhaps it's a cycle. Once a dev team grows over a certain size you are guaranteed to have 25% of your dev staff being under performers and you start to come up with bureaucracy to account for that. The bureaucracy causes some of your A players to leave. You replace them with more mediocrity requiring even more bureaucracy causing even more of your A players to leave...and then one day you wake up and realize that you've spent 6 billion dollars and released vista on an unsuspecting public.

I'm sure big software companies are good at something, but I no longer care to find out what it is. I'm done with bureaucracy and the mediocrity...as soon as I finish this exit paperwork.