Mine was a few things.
As an Xbox 360 owner last generation I could see the direction Microsoft were taking their console, which began putting me off gaming so I sold my console.
Then I watched the PlayStation reveal and I saw instantly that Sony are passionate about gaming, plus their reveal was incredible.
Then I watched Microsoft's reveal, felt my prior beliefs about them were confirmed, and haven't looked back since leaving my 70-something thousand gamerscore and closing my Microsoft account!
I pre-ordered my PlayStation 4 and bought it at a midnight launch. It is a phenomenal console and I only wish I had bought a PlayStation 3!