The aggressive trait gives you a big advantage for conquest. Otherwise, you need something that's going to help you support the costs of a growing empire -- and every trait does that in its own way.
If you want to achieve true conquest, you can't hesitate. You need to beeline to an early powerful unit like horse archers and take out an opponent while they're still getting started. AND you need to raze all but their capitol, otherwise the maintainance costs will choke you. Maintainance will be almost as big a hurdle as any other civilization.
If you hold off on war until the middle ages, it's still possible to achieve domination. The truth is, you don't need a strong UU to dominate or conquest. (But it never hurts to have a good UU.)