The initial investment.
Gaining this initial investment is the hard part, and the biggest hurdle. This investment is something you may have to go without for a long time The investment is a long term one or ideally should be due to the fluctuations of the market. The price may fall as well as rise. In the event of a fall in the price you need to be able to bare the financial burden and not reliant on an income from the investment.
There are also many Cryptos out there to choose from. Which one you choose to invest in is a choice that would require some research before doing so.
How fast the coin can transact and the costs of the transactions should be considered very carefully. The ability to adapt the coin for other uses and the projects being developed on the chain. The scalability of the coin to be mass adopted.
Looking at what the financial markets are doing and the exchanges volume of trading of the coin and the direction it is going in.
This is only my theory. I am not a financial advisor and I know less about the Crypto in's and out's than most. This is the things I have heard others say over the course of my time on the Steem Block Chain.
The main obstacle now is the relative stability of the political and economic system, most people being satisfied with the means of payment at their disposal (fast and simple).
But the situation will change when the population loses confidence in the financial system and the currency, which is sure to happen.
What will happen when Italian banks run out of cash? what will become of the euro if Italy is defaulting on its debt? Salvini having already warned that he would not accept a troika as with Greece.
In case of no-deal, will the Brexit drive the UK into a recession? what impact would this have on the rest of the already fragile euro area?
And, finally, perhaps most importantly, what does the reversal of the yield curve mean?