After building a settler, I often forget to enter the city and create a new army, and then skipping the turn completely forgetting about the settler and thus unintentionally delaying my own expansion. For me, it would be a big improvement if the settler unit would start as an army, rather than inside the garrison. Actually, this applies to all units, since I almost never build a unit in order to improve my garrison - I build it to move out in the world.