I find A to be absolutely unforgivable. The player should always be allowed to play exactly as they desire, and any effort of the GM to manipulate their actions should be discouraged. The only thing the GM should be allowed to give is "suggestions". However, no requests. No scripts. The GM should not know what his or her players shall do next, nor attempt to control it. So I feel.

As for B, I prefer EVERYTHING to be kept as a surprise. The GM should make all the decisions for what will happen in the world him/herself. The players should only know as much as the player characters know, or the experience will be diminished. Someone who states out in the open what will happen in the OOC before it has happened... I want to strangle such people. Really.

Though, as for C, I am entirely alright with a prophecy about a certain individual being the giant-slayer. A specific character can say he or she will be the one to kill the giant, no problem. However, after that, the GM shouldn't control the actions at all. If he ends up being the one killing the giant, wow, nice prophesying, seer! If it was someone else that killed the giant, whoopsie. Saw a bit wrong, eh?

That is my thoughts.