either high iso noise or shutter blur. shooting f6 or less and still running +6400 iso at the lowest shutter speed I could go. you know the deal.
Just some things to bear in mind:
1. Wide lenses will give you DoF at wide aperture.
2. Street lamps, brightly lit shop windows and adverts, car lights etc are your friends.
3. Exposure compensation is your tool - think of stopping down if there is some side light - and you get your drama king.
4. Topaz DeNoise is magic with Nikon files if you do not overdo it.