Barcelona and Istanbul: just be on the lookout for scams in clubs. Google it if necessary.

Shanghai: we went to a few bars/clubs in Shanghai and did not have any problems at all. We were mid-30's at the time and were probably median age for these places...I don't know if Shanghai is a good college or recent grad scene. It's becoming a more expensive town, and liquor is expensive, so maybe not. Or maybe they were just at different clubs. In any case, it's a great city.

HKG is going to be expensive, although very international and totally worth it if you can afford it. We spent more time in upscale bars than clubs there.

I love Montreal in the summer but honestly haven't really partied there. Sizable student population so I imagine it's pretty good...presumably you'd go in Sept/Oct when students are in session but it's not yet freezing cold.