By now we know that Harry's dreams are never unimportant but mostly kind of foretell things connected to Lord Voldemort. I'm reading PS/SS again, and noticed that during his first night at Hogwarts, Harry had the following dream:
He was wearing Professor Quirrell's turban, which kept talking to him, telling him he must transfer to Slytherin at once, because it was his destiny. Harry told the turban he didn't want to be in Slytherin; it got heavier and heavier, he tried to pull it off but it tightened painfully - and there was Malfoy, laughing at him as he struggeled with it - then Malfoy turned into the hook-nosed teacher, Snape, who's laugh became high and cold, there was a burst of green light, and Harry woke.
This dream actually contains quite a lot of foreshadowing - all of the element are connected to Voldemort, but at that time, Harry was unaware of that fact.
1) The high, cold laugh. It's chracteristic of Tom Riddle/Voldemort, but Harry didn't get to know it until years after that dream. Nobody else laughs that way.
2) Quirrell. At that time, Harry thought of Quirrell as a strange but rather friendly bloke. Yet he and his turban (!) appeared in that nightmare.
3) The burst of green light - Avada Kedavra.
4) The Slytherin destiny - the fact that Voldemort transferred some of his powers and qualities to Harry.
And then there's Snape. The dream was before Harry even talked to Snape, before he found out about Snape's hatred for him, before he suspected him, even before he knew Snape was head of Slytherin. So why that association? Does it have a meaning that Snape appears in a dream all the other elements of which are not only connected to Voldemort, but turn out to be threats to Harry? Is it just because Snape once was a Death Eater? Or can it be a sign that Snape's loyalities are not what they seem?
(Or am I reading too much into things?)