I wouldn't say that NYC is uniformly perfect for biking throughout the year. The summer is very hot and humid, effectively requiring you to shower before going to work. The winter is cold, and the roads and frequently covered in snow and/or ice.
Both of these problems can be worked around, but they are daunting for anyone who's not already fairly dedicated; a newbie isn't going to ride to his law office in 95 degree weather, or through slush.
(And that's just the weather; I'm not taking into account the crappy car traffic through most of Manhattan, or the huge valley around 125th street, or the fact that going across the Brooklyn Bridge is incredibly annoying whenever pedestrians are present, etc.)
Digging through [0] (and matching my experience), NYC appears to average summer highs of mid-80s and winter lows in the 20's There are plenty of days outside of those ranges in Feb and August, but there are a solid 6 months/year of reliably comfortable cycling weather.
As to your footnote, both of your geographic problems are avoidable by knowing the city better. Both the Hudson River Greenway (low) and Riverside Drive (high) provide very convenient ways to avoid the 125th St Valley on the west side, and St Nicholas Ave does the same on the east.
Further, what self-respecting (and sane) cyclist would take the Brooklyn Bridge when the Manhattan bridge is __right__ there and in every way nicer to ride on?
Both of these problems can be worked around, but they are daunting for anyone who's not already fairly dedicated; a newbie isn't going to ride to his law office in 95 degree weather, or through slush.
(And that's just the weather; I'm not taking into account the crappy car traffic through most of Manhattan, or the huge valley around 125th street, or the fact that going across the Brooklyn Bridge is incredibly annoying whenever pedestrians are present, etc.)