"What?" and "which?"

There is a clear difference between "what?" and "which?" when they are used as question words. Consider the following examples to try and identify the difference:

What do you think about that programme?.
What is your opinion?.
What media do you know well?
What/Which media are you going to use for this campaign?

Which do you think has more impact: television or cinema?
Which is more efficient: banners or pop-ups?

Indeed, "What?" is used when you ask an open question, while "which" is used when there is a narrow, more limited choice, an (implicit) list to choose from.