Dustin Browder has addressed the autocast concerns

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Many people had concerns that autocast would make the game too easy, removing most of the multitasking generally required to be proficient at RTS games. Dustin Browder (Cavez) has addressed autocast on the Bnet forums:

I have seen a lot of concerns on this board and on other boards about autocast and how this will remove a lot of skill from the game. This would be a really exciting debate, if only we were using it as much as some folks imagine we are. I’m afraid that most of our abilities aren’t really appropriate for auto-cast. Here are some examples from the latest build.

Psi Storm: this ability fires with so much authority, uses so much energy and hits your own units if you’re not careful. The player MUST choose when to do this. We cannot possibly do it for him. Any decision we would make would certainly be wrong most of the time. Plus if we could figure out a rule set for autocasting Psi-Storm (and I don’t see how we could) we would remove way too much skill by putting it in the game.

Force Field: same deal. Position is so important we couldn’t possibly choose for you and be smart about it. Plus it would ruin all the fun of using the spell.

Time Bomb: same. We can’t make it auto-cast. There is no simple rule that would work. Plus, it would remove a lot of skill of using it at exactly the right moment.

StarCraft special abilities are very different from special abilities in other games. They fire off occasionally, they can be used with great precision and they cost a lot of energy so a “miss” is a huge loss. With this kind of design on our abilities, autocast simply doesn’t work very often.

The only place we are doing auto-cast at the moment is on the SCV repair ability and on Medics healing people. Both are very similar abilities. They are used a lot. They have a very low cost associated with them so if the unit automatically heals or repairs something you don’t want, it’s not that big a deal.