The number one reason I'm not even considering it is the size of the phone. I want to use (not experiment with) Windows 10 mobile. I want Continuum. I loved my Lumia 520, I'm jazzed about my 735 (although I am still kicking myself for not waiting to get the rm-1039 model), and I actually like how the 950 looks. The whole larger, flatter trend of phone in general is just not what I'm looking for though.
I held a Lumia 435 the other day, and it felt great! If we do see a Surface line of phones, I'm praying that one of them will top out at 4.5" screen, but keeps all the power under the hood.