There are still many challenges to be resolved by the community.

Miner cartels are becoming greater threat than 51% attack.  Miners need 2 million dollars a day to pay electricity bill. When block rewards decrease miners need higher fees. Miners cant make money unless they reject low fee transactions.

Some of the greatest problems facing Bitcoin
- Governments can determine who is using Bitcoin from IP data. Some governments do not want citizens to have access to technologies like Bitcoin. The identities of the people running the Bitcoin client are not protected.
- Bitcoin only supports 8 transactions/second and Visa's network clears 2,000 transactions/second
- Bitcoin transactions are too easy to track. Bitcoin does not offer the privacy required for widespread use in many countries.

These problems are being worked on.