Perhaps I'll name the most important one: I can't save at any moment and do something else. And why would I even need to explain that? I assume you know the differences between single-player and multiplayer better than I do. You can even tell more drawbacks than I can. But, of course, I ask you to consider everything from the perspective of a single-player's comfort. In other words, if something that exists in the single-player game is missing in the multiplayer game, that's a drawback.
P.S. Please correct the signature. It should be written as "StarCraft II".