I think a useful although grandiose feature would be to allow SO to run posted code snippets inside a secure sandbox (for various languages) for the purposes of: finding syntax errors, validating code, sometimes producing and posting actual results or output.
I would find this feature useful because I don't have all compilers frameworks and tools installed for which I would like to quickly try some of the unique stuff I see posted on SO. Also source code answers could be validated at deeper levels for weighting usability, etc. Also it might be great just to see the immediate results of code, inline with the posted answer, rather than mentally processing it.
The words "results" and "output" are used loosely here and could be anything from a parse tree to a step-through of code to actual console text or anything in between, whichever representation best suits the situation at hand. It could be automatic or chosen by the poster.
This feature request might seem really "out there" but it's fun to speculate about (well, for me). With recognition a lot of thought and work would have to go into it, but anything is possible if desirable.
One of the biggest challenges would be the fact code is often a fragment so running the native compiler would produce lots of errors. However syntax highlighting on SO is already a step in the right direction because it proves recognition of language features to a degree. Missing code might be mocked by a tool.