Lots of things. 
-iOS was getting stale for me.  The update from ICS to JB is what sold me.  I think at that point Google proved that android could be just as smooth and fast as iOS and is capable of so much more.  
-The larger screens with the same or higher PPI than the "retina" displays drew me in.  
-The nerd in me!  I was finding that the iPhone just wasnt as usable for me if it wasnt jailbroken.  I was noticing, and have now confirmed, that almost all of those tweeks I had installed through Cydia came stock or could be added without root on Android.  I am unlocked and rooted now though and just flashed my first kernel last night.  
-The pace at which android is moving is incredible.  You don't get that with Apple. 
I came from the 3GS and then the 4S and loved them both.  I was, and still am, an Apple fan.  But I think it was time for me to try the other side. I couldn't just sit on one side of the fence.  I also can't say that I would never go back.  Depending on what Johny Ive does with iOS over the next couple of years, I may go back.  For now though, I'm here to stay.  I love android.  I love that I have experience with both sides now and feel I can offer better advice to people and be less biased.  
TL;DR Time for change
Jesse