Over the course of 20 minutes, the maximum number of people who can engage in transactions is somewhat under 50,000. This is obviously unworkable as a global currency; if the user base and transaction volumes rise, you might be waiting hours or days for a transaction to complete.
https://en.wikipedia.org/wiki/Bitcoin_scalability_problem
The blockchain also records details of your transactions forever, and unless you carefully obfuscate your activity, every bitcoin user will have the entire blockchain and your entire history on it. Forever.
https://en.wikipedia.org/wiki/Privacy_and_blockchain
This certainly gives me pause.