-quality steel frame with high bottom bracket, short wheel base and aggressive "tracky" geometry in 700c
-slim bullhorn bars
-put some brakes on that and a grupo according to topography, climate and special needs. If your city is hilly, single speed is not recommended, otherwise it is.
-bombproof rims like DT Swiss 521 or Rigida DP 18 coupled with bombproof tires like Schwalbe Durano in 25mm or 28mm
-comfortable slim saddle, I like charge spoon, but thats my ass.
Inna city you want a light, minimalistic robust bike that rides fast and responsive on asphalt and in traffic, at the same time you need rims strong enough to take their daily curb stomping and tires that can wheel though broken bottles all day.