What gameplay behaviors are random?

Interesting question. I can think of the following (with 1, 2, 3 and 5 already mentioned by you):

1. Starting locations.
2. Choice of opponent(s).
3. Map choice, if it's random.
4. Race choice, if it's random.
5. SCV movement while building.
6. Larva movement.
7. Larvae location after injection is done.
8. Creep spread around a creep-generating point.
9. The time it takes to burrow and unburrow.
10. Interceptor movement pattern (take a look at this video - if it wasn't random, all interceptors would have probably taken the same route after the first attack, but they don't).
11. Also, I'm pretty sure there's also a small variance on the viking transformation duration - see this related question.

In any case, while items 5 to 11 do have some concrete gameplay effect, I'd say their impact is very low.