Diagonal proofs like the Halting Problem say that in general you can not prove properties about all programs in a Turing complete language. Thus, there is a class of troublesome programs.
Likewise there is obviously a great number of programs in Turing complete languages that you can prove properties about. Thus, there is a class of pleasant programs (or [program, property] pairs).
We do not know for certain how many of the programs that we want to write that is in each class. I would venture a guess that >99% of all programs ever written are in the "nice" class. If it is really impossible to reason about some property of a program, why would you think it works?
