For me the core of TextMate’s power comes from its bundle and scope systems. Allan has a little bit to say about that in the segment of the interview about sources of inspiration:
A big non-editor inspiration was CSS selectors which is what I recreated as scope selectors. The first time I read the CSS specification I was pretty excited to try out the concept. Unfortunately I did not have access to any browser which implemented it, so I started writing my own implementation, though I never got very far with it. Still, a seed had been planted and on an unconscious level I have probably tried to find a place where I could implement them, ever since.