>>10
TeX is Turing complete. It's also a complete pain to program in. Attempt to write a macro to do anything useful that should be fairly simple: you'll wish you hadn't, because you won't get your time back.
If you're really up for a challenge, try making it write out your source file with all tokens expanded to primitives. You'll then learn how irritatingly context sensitive TeX (and LaTeX) can be.
LaTeX also doesn't really separate math notation from math meaning, which math web standards like MathML, OpenMath do. The ability to independently include other standards for semantic web, scripting, web support, style transformations, etc, also makes web documents highly flexible. A preprocessor in a better language that (truly) separated meaning and style and output to such formats could really be something.