Since this topic is sort of centered around a small part of my experience, I'd love to shed some light on a couple points:
True! I started my own business! For about 7 hours of my life I didn't know what I was going to do if I couldn't bloom at 37signals, and then I told David and Jason I was starting my own company, CoSupport.us.
True: I left 37signals because I really wanted to do more support-wise, and the needs of the company didn't mesh with my ideas. They need people full-time on support, they don't need someone who's not there responding to tickets. After 4 years of answering 100+ emails a day, I wanted to do other things. Needing me on tickets interacting with customers is what Jason means when he says they can't have people working on a team not doing the work of the team.
True: David and Jason may sometimes come off a bit stiff, perhaps even cold, when it comes to how they manage their business, but that's the unfortunate aspect of print, edits, etc. In truth, working for them was the most challenging, educational, inspirational time of my career so far. They are the opposite of cold when it comes to their employee's ambition, ideas, talent and goals.
True: My experience leaving was equivalent to a professional football player saying to his coach, "Hey I think I want to try being a referee!" The player isn't on the team to be a referee. If he wants to be a referee, he needs to go to referee school and become a referee, not say, "I'm going to stay on the football team and be a referee, too." That's not how it works in football, or at 37signals.
False: I was not at all "rewarded" for my ambition by being shut down, put down, held down, or in any way shuttered by David or Jason. I was rewarded by them telling me I had too much ambition to be contained there, and by them supporting my new company as best they can.
False: I actually did my first job for 37signals in 2005, and started full-time in March 2007. So a little more than "just about 3 years."
The only bad thing I have to say about 37signals or their management or my leaving is that they stole my frog, which you can ask Jason Fried about.