Not that this was necessarily better in terms of capabilities, but TextMate had a very pleasing Unix-style extension model where there was no mandated language and extension commands used scripts/executables written in any language. There was even a nice graphical editor for fine-tuning exactly what input they would be given and how their output would be acted upon.

TextMate was very much "Mac OS X UI sensibilities combined with Unix power", whereas ST pretty much has its own self-contained philosophy that's then brought to Mac/Windows/Linux in a slick way.