I think it's a great question with a very simple answer. Areas that are not prime, will see the largest downturn. Areas which are prime, will remain prime. Upper East Side is voted one of the best neighborhoods to raise a family. Tribeca was voted by Forbes as the most expensive zip code in New York and I believe America, West Village has everything, Soho, Bowery, Chelsea, Meat Packing these areas are going no where. Most will dip, but very minimal.
It is a fact. This city has occasional dips for a two year period every ten or twenty years. The nthings continue to rise over the remaining period and a SMALL dip again for a year or two.
There are some areas which will and have never fallen far.
Anything along a park. Central, Madison, Gramercy, Washington Square come to mind.
Anything which has exceptional value.- Superior design, amenities, natural light, views (everchanging in this city, so think outside the box here), location (prime subway access like Columbus Circle or Union Square)
The Village has always perfomed well. With the Sex and the City movie coming out, I know it will out perform many other areas when we see a small dip in the third and fourth quarter this year.
Gramercy is a stud! It is the area where people may look for years just to get a key to the park.
Chelsea is another area that should hold up well.
UWS is a timeless classic that with two different trainlines (IND and IRT) will always hold strong as it is bordered by two great parks.