"What good could possibly come from watching?" This means that there could be some harm from watching, but probably not anything good can come from it.

You're questioning how bad a movie or TV show will be. Then you're thinking it can'tbe that bad.

The first one is grammatically an exclamatory sentence, not a question but to express strong feeling. Because the modfied word "good" actually means "good things" as a noun, grammatically "what" should be used. e.g. what fine weather. what a beautiful girl. what beautiful flowers the second is a clause of comparison, "as…as…" should be used when you comparise things at the same level. The first "as" is adv, to say the adv or adj behind modifies things at the same level, not lower or higher. the second "as" is conj, followed by things you want to comparise with, and the two things be comparised should have same part of speech. Here "what I am picturing" is comparised with "it", they could be regarded as noun. My English is also limited, but i wish i could help u grammartically from the point of view of an eastern learner.