The original Bitcoin protocol is very innovative. However, there are several reasons for why today's cryptocurrencies will fail:

1. Too slow transaction times for general use.
2. Too bloated block chain for mainstream adoption.
3. Having to store coins "under the mattress" is a step backwards.
4. Proof of work will lead to too large transaction fees.
5. Proof of stake leads to the rich getting richer.
6. Lack of government support prevents mainstream use.
7. Trading against ordinary fiat currencies causes volatility.