It was a particularly bad string of basic package incompatibilities that woke me from my emacs evangelist phase, but it was the realization that emacs was downstream from TextMate on several non-GUI-centric features that made me realize the entire "platform" value prop was bunk. While emacs might be excused for missing tectonic shifts (e.g. away from CLI and towards GUI), entirely text-centric features like multiple arbitrary simultaneous caret, Smith Waterman quick file/action search, powerful template substitution, parsing-based syntax highlighting, navigation, and refactor tools all developed and matured outside of emacs and then slowly and painfully found their way back in, not the other way around.
Emacs was supposed to be a fertile platform for text editing innovation, but history didn't work out that way, even if you ignore the trend towards GUI innovation, which you shouldn't.