SC2 is a deterministic game in the sense that there is nothing left to chance. No units get a "20% chance to cause bonus" or anything like that. The same keystrokes will always yield exactly the same result.
But there are some elements which have an element of randomness. The importance of this is that there are still some elements of the game which cannot be predicted. I'd love to see a list. Here are a couple I can think of right now:
- Starting situation (spawn location, map, ladder opponent)
- When an SCV is constructing a building, it moves in seemingly random patterns during construction. If you're lucky, the SCV will move where he can't get sniped.
There are other random things but don't affect the game, like the appearance of a dark templar (scythe or no scythe).