1) Vayakam melech chadash... asher lo yada es Yosef. Rashi explains that Pharoah pretended that he never heard of Yosef; the memory of the contributions of Yosef were blotted out of Egyptian history. This gave Pharoah license to begin his persecutions.
The Brisker Rav asks why Pharoah had to pretend -- why couldn't he simply initiate his plot against the Jewish people in spite of what Yosef had done? I think the answer is that the anti-semite has a psychological need for self-justification. To accept that Yosef had done good and at the same time want to destroy the Jewish people would create uncomfortable cognitive dissonace. The first step has to be to demonize, to define the victim as an outsider, as the "other" who is worthy of persecution.
2) On a different note, when Moshe returns from Midyan with the message to Pharoah to release Bnei Yisrael, he does not go straight to Pharaoh's palace, even though the fact that he could enter Pharoah's most private space umolested (according to the Midrash Pharoah had lions guarding his garden and Moshe was able to miraculously walk right past them) would have given instant credibility to his message. Instead, he first gathers the leaders of Bnei Yisrael and relays his message to them. R' Chatzkel Levenstein writes that geulah does not begin with miracles; it begins with belief. The buy-in to the message has to happen before the miracles can begin to unfold.