In NYC you won't have the cold like we have in MN. IGH are nice in the winter, but at least my SA IGH case, they also don't always shift well when it gets much below freezing (I'm going explore lube options for next winter). Disk brakes are great. Upright and somewhat wider handlebars are great. I prefer straight or straight-ish bars to North Road type bars in winter. If you have much ice (I can't recall how much problem ice was when I lived in NY, too long ago) studded tires are good. Wider tires are good, and aggressive tread is good if you will ride unplowed or under-plowed streets and for surviving road hazards.
Clothing-wise, you won't need to overdo it. Even in MN (below zero air temps are common, double digit wind chills even before you create your own wind chill on the bike) I never need more than long underwear, an sweater and a wind shell for my body. In NYC ski gloves will work (here in MN bar mits or mittens are a must). A nomex face mask is nice to have for colder days (and everybody will get out of your way when you walk into the bank!).
Gears are good if you ride in snow I think, it's best to spin, but lots of folks make do with single speeds here too.
Your mountain bike sounds like it's mostly there for winter suitability. What about the geometry doesn't please you on the road?