I think that SC2 should be studied for its qualities but also for its mistakes.
I think it is overall an excellent game but at the same time I feel that it was not as well designed as SC1, even if patches improved it and fixed some glaring errors over time.
A few things that come to mind:
- Free units, they are extremely hard to balance and can easily lead to stale situations, and in my opinion they never feel right. And to be clear, projectiles are not free units.
- Spell casters able to stand a fight on their own and be massed, again it does not feel right and lead to imbalance/abuse. Spell casters should be support units, fragile and not scalable to a whole army.
- stackable aoe damage, because it leads to death ball armies that can be extremely difficult to engage, with somewhat difficult to read and seemingly random outcomes. A good aoe design is the SC1 inspired Psionic Storm, powerful but not stackable.
- Hero units, there is only one hero unit in SC2, for no good reason.
- Population cap, in SC2 the population is capped at 200 supply, this is an inheritance of SC1 technical limitation, but it has a strong impact on SC2 late game, not for the best as players tend to invest in defensive structures to compensate, leading to potentially static/ boring games.