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?
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson