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C: Cross-Platform PDP Assembler
The C language was written to enable UNIX to be portable. It's designed to produce good code for the PDP-11, and very closely maps to that machine's capabilities. There's no support for concurrency in C, for example. In a modern language such as Erlang, primitives exist in the language for creating different threads of execution and sending messages between them. This is very important today, when it's a lot cheaper to buy two computers than one that's twice as fast.
C also lacks a number of other features present in modern languages. The most obvious is lack of support for strings. The lack of bounds-testing on arrays is another example—one responsible for a large number of security holes in UNIX software. Another aspect of C that's responsible for several security holes is the fact that integers in C have a fixed size—if you try to store something that doesn't fit, you get an overflow. Unfortunately, this overflow isn't handled nicely. In Smalltalk, the overflow would be caught transparently to the developer and the integer increased in size to fit it. In other low-level languages, the assignment would generate an error that could be handled by the program. In C, it's silently ignored. And how big is the smallest value that won't fit in a C integer? Well, that's up to the implementation.
Next, we get to the woefully inadequate C preprocessor. The preprocessor in C works by very simple token substitution—it has no concept of the underlying structure of the code. One obvious example of the limitations of this setup is when you try adding control structures to the language. With Smalltalk, this is trivial—blocks of code in Smalltalk can be passed as arguments, so any message call can be a control statement. In LISP, the preprocessor can be used to encode design patterns, greatly reducing the amount of code needed. C can just about handle simple inline-function equivalents.
The real problem with C, however, is that it's the standard language for UNIX systems. All system calls and common libraries expose C functions, because C is the lowest common denominator—and C is very low. C was designed when the procedural paradigm was only just gaining acceptance, when Real Programmers used assembly languages and structured programming was something only people in universities cared about. If you want to create an object-oriented library on UNIX, you either expose it in the language in which it was written—forcing other developers to choose the same language as you—or you write a cumbersome wrapper in C. Hardly an ideal solution.