Amid train tracks, old warehouses and derelict homes (it was too close to the Wall) you find a big chunk of Berlin’s nightlife, specially the ultra-famous Berghain. Karl-Marx-Allee is quite an attractive Soviet-style boulevard and makes one feel in Moscow. The area is still under development and rents tend to be cheaper than in other central parts of town, so there is a good mix of creatives, families and foreigners. Warschauer Strasse station in the evenings is quite an attraction, with live music performances, lots of beer around and the promise of an amazing night out. To burn calories or recover from the previous night, the Volkspark Friedrichshain is a good option.
My favorite areas: Oberbaumbrücke, Karl-Marx-Allee and the RAW Gelände area.