One of the first things you notice is the frenzied activity - lots of honking horns and people talking. Something else you notice occurs while waiting to cross the street. The native New Yorkers know exactly when to start walking (meaning before the light turns) while visitors like me wait until the sign says walk. This is a way to tell who is a local and who is from out-of-town.
Everything is narrow, restaurants, shops, even the hotel rooms. But since Manhattan is an island and one of the most expensive places on Earth, it makes sense. I was able to find a nice little deli type shop to get an inexpensive lunch ($10) named Le Mirage Cafe (on W. 43rd Street). No such luck when it came to dinner - everywhere was expensive.
Something else that I noticed about Manhattan is how bad AT&T's cellular service is. Verizon & Spring worked well, but I came across several people who simply could not make a call using their AT&T phones.