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.
What does this mean about "under the mattress"?
That's the biggest problem! Cold storage. An absolute nightmare to have manage oneself even for a crypto-savvy person. Some people may enjoy storing private keys on paper etc, but those are the fanatics. For ordinary people the mess with having to secure the coins oneself is a major step backwards, into the Dark Ages.