The main problem of Bitcoin is its vulnerability .. by that i mean it is too easy for a skilled hacker to take out coins from people who are new to it or even average users too and when looking from the other side it is also vulnerable as the rates literally fuctuates about 40% every month ..this is something that is beneficial for the people who are into trading but for the rest of the people who use it for other purposes rather than trading might have to face the worst circumstances.
^^ This! Security is still the main problem and the biggest challenge for the mainstream adoption in my opinion. People just suck in online security, that is proven numerous times. We need to progress a lot in this field if we want the average Joe to put his hands on bitcoins. I mean, Bitcoin is not bank that they will reimburse you if you get hacked. Here, when you screw over you have screwed and lost your coins.