I saw the other thread on targeting and it seems people don't want to make it easier but I'll throw this out anyway. Double clicking a skill should use it on the closest enemy or ally depending on what kind of skill it is. For the dual purpose skills it just targets whoever is closer.
What about skills you have to aim?
Meh, doesn't seem like it would be used, or it would be used by lazy noobs. Targetting the closest hero doesn't usually mean you're targeting the best hero