The U.N. is expressing fears of the Egyptian Revolution. Hosni Mubarak has been an ally of the United States and has been a very powerful force against international terrorism. His administration, over the past 30 years, has also been a key defender of Israel. That being said, he is without-a-doubt autocratic. If the people succeed in overthrowing his government or in forcing him to resign, there could be a huge threat to other moderate countries.
In a national survey of Egypt, over 50% of the population considered themselves to be fundamentalist and not fond of the U.S. government and people. With the Islamic Brotherhood, a radical Muslim group, backing the people and their protests, a possible new form of government could emerge with the military. The military may not be stopping protests, gatherings, or curfew-breakers, but they are not object to using force to stop an overthrow in opposition to their wants.
The Islamic Brotherhood has been repressed by Hosni Mubarak despite their power and will. These events in the Middle-East and Northern Africa is an opportunity for them to emerge. Their power has potential to lead them to claiming the reins of the revolution. Examples from history prove it to be possible: Lenin and Stalin with Russia, Castro with Cuba, Napoleon with France, etc. The arguments against that are the examples of most of Europe and the U.S.
Both outcomes are possible, but the worst is being expected by some. The citizens of Egypt, Tunisia, Yemen, and other countries have been oppressed and kept in poverty by those in power; them settling for anything better or a majority opinion which could be radical is legitimate (according to the national survey).
With our very powerful ally on the teetering point, the U.S. watches carefully, torn between the democratic ideals of the people or the power of the government. It offers no ability for an accurate assessment as to which direction either side could lean towards.