It's a good question, right? Why are the markets - particularly the US markets - worried about the current unrest in Egypt? After all, it's a fairly small country that stopped being a major player in international politics round about the time that Augustus Caesar made it the personal property of the princeps, and it only really achieved any contemporary international importance with the completion of the Suez Canal. It's not like we're on day seven of riots and protests over the government in China, after all.
I'm not going to profess to have all the answers here, but this will at least give you some information to work with.
First off, there is the Suez Canal, through which passes about 7.5% of the world's sea trade. If the Suez Canal were to shut down - say, if there were open civil war in Egypt - that 7.5% of the world's sea trade would have to round the Cape of Good Hope instead of taking the nearly straight-line 120 mile canal route between the Indian Ocean and the Mediterranean Sea. Rest assured that this would cost shipping companies (and, by extension, companies that use the shipping companies, which would then get passed on to wholesalers, then to retailers, then to consumers) far more than the current average $250,000 per ship that it costs to traverse the canal.
Along with the Canal there is the SUMED Pipeline, which carries roughly 1.1 million barrels of crude oil per day north from the Red Sea to the Mediterranean Sea. If that pipeline were shut down, that oil would either not go anywhere, or would have to go by ship around the Cape of Good Hope. Now, since the average supertanker can hold about 2 million barrels, so that doesn't seem too bad at first glance. But, since that would require another 180 or so supertankers rounding the Cape of Good Hope each year (each one burning petroleum in their own right), that starts ramping up the cost of oil in the blink of an eye. And that's assuming none of those tankers experience any problems.
But that's just part of the problem. Have a look at current private foreign investments in Egypt. There are four major western oil companies operating in Egypt (BP, Eni, Apache, and the BG Group), 11 western banks (Barclays, BNP Paribas, HSBC, Societe Generale, Bank Audi, BLOM Bank, Citibank, Credit Agricole, Intesa Sanpaolo SpA), MashreqBank, and Piraeus), a major European retailer (Carrefour), two western telecoms (France Telecom, and Vodafone), and a major western manufacturer (Nestle). In the event of a civil war erupting, many (if not all) of those companies would pull out, which would have a brutal impact on the Egyptian economy and a substantial impact on the companies.
Interestingly enough, Egypt's current troubles are also reflecting on perceptions of risk for investments in Saudi Arabia. Both nations derive primary revenue streams from oil production, have aging heads of state, and deep social divisions. They aren't identical by any means - Saudi Arabia has far more wealth, for one thing - but they're similar enough for Egypt to provide a prototype of what could happen in Saudi Arabia. Also, since Vodafone caved in to Cairo's demands to cut internet service in Egypt, that has analysts concerned about whether or not other ISPs would do the same under political pressure from other nations.
Also, and this is purely speculation on my part, civil war in Egypt could escalate into a broader regional war. If a broader regional war starts, then Israel (which borders Egypt and has fought a war with Egypt less than 50 years ago) will get dragged in. If that happens, all bets are off.
 That we know of, anyway.
 About 10% of that sea trade is oil tankers.
 This adds about 6000 miles to the distance traveled.
 Also, the SUMED Pipeline is only currently running at about half of it's 2.5 million barrels per day capacity. If that picks up, that would be one supertanker every day.