Hosni Mubarak is a dictator. He is not good for his people. As a liberal, I should be delighted at the prospect of his overthrow, but I'm not. Who will replace him if the mass riots bring his downfall? Probably not the mild-mannered Muhammad El-Baradei, former chief of the International Atomic Energy Ageny. He isn't very popular in Egypt, despite the fact that he's the opposition leader who is most prominent in Western media. The most popular movement in Egypt is the Muslim Brotherhood, the undemocratic fanatically Islamist older brother (or father) of Hamas.
Having Hamas ruling Gaza is bad enough. Having a Hamas-like movement ruling all of Egypt, together with the prospect of Hezbollah ruling Lebanon, is quite a scary idea for us Israelis. But in a way, the Muslim Brotherhood in Egypt would be worse for Israelis than Hezbollah in Lebanon. We don't have a peace agreement with our northern neighbors anyway. With Egypt, there is a risk of this important Arab country rejoining the ranks of our enemies, and actively arming Hamas against us.
Egyptians, if you get rid of Mubarak, please don't replace him with religious zealots!