Syntax highlighting is an indespensible feature of a programmer's editor - I don't know how I managed without it and always miss it if for some reason it doesn't activate.
One aspect of syntax highlighting that editors don't seem to do very well, however, is multiple levels of meaning. For example, if I have some code that's commented out the "top level" of meaning of this text is that it's a comment. The next level underneath that is that it is code. Syntax highlighting just treats it as a comment, however, and makes it a single colour. It would be neat if, within a comment (or other section that can be interpreted on multiple levels) the editor would try to parse the text and tweak the colour based on that. Commented out code would then be syntax highlighted, but would also be tinted green to show that it was within a comment. Similar techniques can be used for "#ifdef 0"ed out code, code within macros and code within literal strings. This would make it much easier to work with this sort of "multi-levelled" code.