Keep in mind that most Egyptian workers expect tips after performing a service, known as Baksheesh. This can be expected for something as little as pressing the button in the elevator. Many workers will even ask you to tip them before you get a chance. The typical tip for minor services is 50pt to 1 LE. Due to the general shortage of small change, you may be forced to give 5 LE to do simple things like use the bathroom. Just understand that this is part of the culture; the value of the baksheesh is very small to most westerners (USD$0.10 to $0.25) but makes up the a good portion of monthly income for many Egyptians.
Do not photograph people without their permission, and in areas frequented by tourists do not be surprised if a bit of baksheesh is requested. If you're male, don't be surprised if another male holds your hand or forearm or engages in some form of bodily contact - there's no taboo against men holding hands and unlike in the West, this behavior is not associated with homosexuality. In general, Egyptians are a lot more comfortable with less personal space than are most Westerners; however, pairs of Westerners should be cautious in engaging in same-sex contact. Normal contact is quite acceptable (shaking hands, pats on the shoulder, etc.) but holding hands could be mistaken in Westerners as a sign of homosexuality, which is quite taboo in Egypt. Smoking is very common and cigarettes are very cheap in Egypt.
Gamal Abdul Nasser, the first President of the Arab Republic of Egypt, and many others are considered national heroes in Egypt; you should say absolutely nothing that could be perceived as offensive or derogatory regarding him. Hosni Mubarak (the current President) is largely unpopular but it is probably better not to discuss politics unless your Egyptian acquaintance brings it up first. Tread carefully around such topics and let others guide the openness of the discussion. Many Egyptians have a different interpretation concerning ambiguous expresions such as freedom of speech and democracy. Likewise, don't bring up politics and other delicate issues impulsively. It is advisable not to discuss Israel even if tempted; do not speak loudly about it as it may attract unwanted attention, even if you are only talking about it as a travel destination.
Never discuss religion from an atheistic or similar point of view. Even highly educated Egyptians who studied abroad won't appreciate it and doors will close for you. Also be aware that the Islamic "call to prayer" happens five times daily and can be heard loudly almost anywhere you go. Just understand that most Egyptians are used to it and enjoy it as part of the cultural experience.
Take great care if you choose to drink, especially if you're from countries where heavy drinking is accepted. Even if you are used to it, you can't estimate the effects of the climate, even at night. The impact drunk people have on Egyptians is quite large and very negative. The best plan is just to abstain or limit yourself to one drink per meal while in Egypt; it will be cheaper too.