More than gears, I'd be thinking about the coaster brakes.

Single speeds are great fun, and more versatile than I thought they'd be. (with the right gearing!) Can't say I really missed having gears while toodling around on a sweet smooth lugged steel single-speed mixte.

But think about how you want to brake.

Feet or hands? What's your instinctive reaction if you need to stop when a kid wanders in front of your bike?