Btcs volatility is the main problem. In a deflationary state, where the value of Btc rises, people will tend not to spend their coins as they would get more a day/week/year later. That transition from asset to currency might be too difficult for mass adoption.
I have to agree with that, though I don't think there is anything particularly wrong with this state of affairs. I often say it myself that Bitcoin right now is rather a financial asset than a currency. But as long as its price increases, it doesn't really matter since it still gains value. Whether this gain comes through speculation (i.e. Bitcoin being used as a financial asset) or through its use as a currency (i.e. Bitcoin being used as a means of payment) would be inconsequential in the end. When the price growth potential of Bitcoin gets exhausted in due course, the volatility will decrease as well...
And this may celebrate the change in Bitcoin use from a speculative asset toward a means of payment, i.e. currency