The primary one is definitely education, a lot of folks are either misinformed about Bitcoin or simply do not know about it. On a related note, a lot of Bitcoin material is language locked (i.e. English only), which locks out a significant portion of the population.

Another challenge is infrastructure related, this spans from electricity issues all the way to Internet connectivity. However, mining will play a huge role in solving the former and the later mesh network and other related internet Infrastructure.

I'd also say there are still few pathways for devs to get from zero to hero, and products and services, while having made great strides, are not at a stage where folks use Bitcoin without knowing it.