If I was forced to pick one of these I would say merchant acceptance, the easier it is to trade your Bitcoin for goods and services, and avoid fees from traditional rails the more people would want to use it, it is just a simple incentive.

But I think while a lot of people have heard about Bitcoin, very few actually know what it is, in the grand scheme of the world, most people just know their local currency and that's what they default to, so for me time for the lindy effect to grow is really the major factor