Ever since I installed HotS, I've been unable to select enemy units. Is this a new feature with the expansion or did is there an option to change that?
This is a new feature in Heart of the Swarm which was introduced in the Beta.
It is default set to not enable Enemy Unit selection.
It can be changed in the Gameplay options.
I found out its an option in the menus. Options -> Gameplay -> Enable Enemy Units Selection
I'm not sure why Blizzard doesn't make by default.