Wasting my scant mobile bandwidth with ads is the main thing that would make me want to go back to a dumb phone. I am sick of people presuming that it's OK to wedge ads into every little nook and cranny in life.
I instantly remove any Android app that I find to have ads, and iAd was the main thing that made me think twice when I was buying the Android device- and put me off the iPhone a bit.
Given that the main use of a browser on my phone is to check the national rail website for train fail, I can easily do that on the little (and excellent) java-based Opera port. Every other whizzo feature on my smartphone was just there because I wanted rather than needed it. If it becomes riddled with adverts, then screw it, it is no longer a device that appeals to me.