While I think NYC can be overrated, pollution is IMO the least of its faults. Personally I think it's paradise compared to HK (much less real pits like Beijing or Cairo) when talking about pollution. Vegas I wouldn't even rate it as a major travel city at least on the international scale, so I would be surprised that it's overrated. Ditto, the likes of Orlando or HNL. LA would be my choice for overrated city in the US.

Someone mentioned SIN as overrated, but again I find it strange that it would be seen as a "popular" major travel city in the first place. Major travel city possibly, major transit stop and biz destination certainly, but "popular"?

Paris and London would be my top contenders for overrated. Dubai in the Middle East, although I'm not sure it's actually popular. For Asia, probably BKK.