A mobile phone that catches my attention the most would have the following features:
GPS: Global Positioning System
Music, video, FM: Something to pass the time when I'm bored
Smart operating system: Lots of things to play around with
WI-FI: A smartphone without WI-FI is just pitiful to even think about, but Wi-Fi base stations are not yet widely available domestically.
Actually, these features alone would be enough. But the G1 has more to offer...