-as an investment, bitcoin is fundamentally a ponzi scheme. someone may argue that it is a challenge only if you are too late in the game, but for me, that is a reason I am not touching it as an investment.
-as a money (store of value and measure of value), there is a fundamental issue that there exists no mechanism whatsoever that would stabilize it's value (against neither a typical consumption basket, nor USD or any other currency)
-as a currency (means to make transactions). You might argue that it is only about implementation that the network can handle a couple of transactions a second. But a widely used currency should handle _lot_ more than that. Assuming one billion transactions a day means roughly one transaction per person per ten days globally. Every single byte you need to store a transaction implies a gigabyte of data per day into the however implemented network. I would call that a challenge beyond just implementation.