Using Notepad++ for most of my coding things, too. First I've started to write web sites with it, then its main purpose became Lua-scripting - and now I'm just starting with writing C++ code
For me, its key features are the almost instant startup time, the inbuilt console and "Compare against SVN Base" - and, of course, the syntax-highlighting
Btw, I've noticed that newer n++ versions come with a function parameter reference and -autocompleting in some languages. Does someone plan to write one for the WoW API? (or has already done it?)