Any use of metaprogramming without having thought damn hard about whether there's a better way to acheive this using the normal, non-'meta' idioms of the language, I tend to find annoying.
An obsession with "DRY" (don't repeat yourself) where some fiendish piece of metaprogramming spaghetti is invoked to avoid repeating yourself, say, twice in a simple and actually-more-straightforward-and-readable-than-the-alternative fashion.
Any use of 'eval' in particular. As metaprogramming goes, this one should be your absolute last resort after trying everything else. eg a lot of rubyists appear not to have heard of Class#define_method.
