The makeup of any given Coptic Orthodox congregation is likely to vary according to the demographics of the surrounding area, though the OO in general are less well-known in the west than the EO, so we do receive fewer converts overall. As AMM notes, however, that is changing, and as more Coptic, Syriac, Ethiopian and Eritrean, and Malankara Orthodox (Indians; usually Malayali people from Kerala in south India, though not always) people are born in the west, the OO Church is becoming less attached to foreign lands, and more able to assert its own identity as a well-integrated part of western societies (one of the first things I was given by a friend when I told her I was finally moving to a place that had a Coptic Orthodox church was a bumper sticker that said "Proud Coptic American" and had the Coptic cross with the American flag next to it...). The Copts are probably known to be the most outwardly/missionary focused (the Armenians the least, though they too do receive converts), and there are places in the world where the entire church in that country is only native/non-Coptic people, with the exception of the priest and bishop (because the church is too new there to have raised one full generation within it). This is the case in Bolivia, for instance, where over 400 people attend services weekly at the cathedral in La Paz, with more in various locations elsewhere in the country.
So I really can't say what you'll find at your local cathedral. Probably a majority of Egyptians, maybe some Ethiopians and Sudanese, Eritreans, Syriacs, Indians, etc. and maybe some "non-OO" converts. The church at which I was baptized had a lot of different kinds of people: Egyptians, Eritreans, Iraqi-Coptic Assyrians (that was an interesting talk...), Hispanics, an entire white American family with no Egyptian connection whatsoever, etc. My home parish in the next state over was all Egyptians (only 6 families when I was there), though some of them were Coptic-Sudanese, and we regularly welcomed people from Jordan, Ethiopia, and apparently since I left the state a few years ago a woman from Saudi Arabia of all places converted and was baptized. We do not exactly have the reputation of, say, the Antiochian Orthodox or the OCA (obviously), but thankfully OO congregations tend to be becoming less and less homogeneous as time goes on.
What this has to do with taking off shoes, I don't know.
Our reason for it is exactly as Fr. Matt has already given: Moses was ordered to remove his sandals because he was standing on holy ground, and because we are too whenever we enter the church or monastery, we do likewise.
