I just fell in love with Vim after years of TextMate (and BBEdit before and Dreamweaver 3 first).
It's powerful, smartly designed, elegant in a weird way… and it works exactly the same on my Ubuntu box at home, on the VPNs I work with and on my Mac at work.
There are many reasons a program wouldn't work across systems. The primary would be that it is a Mac OS X or Windows-only app (TextMate) or that it needs a GUI, or that it makes too many assumptions about the platform it is running on. Vim works on many (if not all) systems, it can be run with or without a GUI without losing or breaking anything, etc. Obviously, Emacs has all the same pros but I liked Vim more.
It's powerful, smartly designed, elegant in a weird way… and it works exactly the same on my Ubuntu box at home, on the VPNs I work with and on my Mac at work.