A Biased and Personal Commentary: Istanbul 2010
One of the first things you have to contend with in Istanbul is the hawkers: they're outside every single store and restaurant in the tourist area and for blocks and blocks around, saying "take a look at our menu," or "best food in town," or "where are you from?" or "can I ask you just one question?" This last is a good starting point for a discussion about rudeness: your choice is be rude now or be ruder later, because it will not be just one question. So the answer is always "No thank you," and this will only shake them about half the time. Just keep walking.
The hawkers are one of Istanbul's worst aspects: I mention them first because they're also its public face, and they really wore on me and made this a less enjoyable visit than it might otherwise have been. I treasure my half-day in Bebek: it's technically a neighbourhood of Istanbul, but it's not frequented by tourists and for four hours, four entire hours, I wasn't solicited once.
Turkey is roughly 99% Muslim (that's the statistics, not added emphasis), so many of the monuments you're likely to visit in Istanbul are mosques. Highlights of the trip for me were The Rüstem Paşa Mosque, Chora Church, and The Alexander Sarcophagus at the Archaeological Museum.
Hagia Sofia, one of the most recognized landmark names in the entire world, turned out to be both amazing and a bit of a disappointment. The scale is spectacular, with the main dome standing 55 meters over the floor. But this Byzantine church was later converted to a mosque by the Ottomans (and the four minarets added at that time), and nearly all of the Christian elements were plastered over. Now it's called a museum and no longer serves any religious purpose. As far as I can tell from photos on the internet, it's been in a constant state of dusty renovation for the past twenty years, and is still full of scaffolding with peeling paint everywhere. They also seem rather uncertain about what they're aiming for with this neverending renovation: they've uncovered a fair amount of the Christian mosaics, but they're leaving up huge wood discs with Muslim calligraphy on them. It will be something to see if they ever settle on a target and get finished.