The language is carefully constrained so that type inference can work in the majority of the cases, allowing efficient code generation. The details are in our julia design paper here.

http://arxiv.org/abs/1209.5145

Also, a number of other language features include: `ccall` to make it easy to call C and Fortran libraries, multiple dispatch which is already discussed here, real metaprogramming capabilities which have traditionally been considered optional in this class of languages, keyword arguments (coming soon), coroutines (julia Tasks) which integrate nicely with the asynchronous networking runtime so that it is easy to overlap computation and communication, immutable types, and I am sure this is not exhaustive.