Other common security problems


Lack of host security
For any sort of crypto to work, you need a Trusted Computing Base on which to perform your computations.  If that's your Windows desktop, you're in trouble!
If it's someone else's Windows box, doubly so!

Humans have disappointingly poor computational ability
People can't do SHA1 or RSA in their heads!
Crypto only provides security from one processor to another; how do you secure the links between the users and the processors?

Social problems are poorly suited to cryptography
There's no "morality bit"; a cryptographic protocol can't distinguish between a "good" use and a "bad" use.