What I think is the greatest problem of C is the confusing syntax for types. Other than that, I think C is good (and in many ways, does many things better than a lot of modern programming languages do), although there are things I would improve (some of which are already implemented in GCC), including a few additional macro functionalities (including adding to existing macros like {append} does in Free Hero Mesh, and a kind of macros which are scoped as variables similar to vardef in METAFONT, and the ability to count the number of arguments to a macro (without expanding any of the arguments), etc)