At exactly 1:30 PM two years ago I was doing my exit interview at Microsoft.
I told the HR person the reason I was leaving was the lack of innovation and opportunities at Microsoft. To my surprise, she said that was the number one reason most Microsoft employees leave.
That was probably my #2 reson, but that's what she cared about. My #1 reason was to create a successful company. Nope, let me correct that. The #1 reason for me to leave Microsoft was to help people create their own web presence.