To put it frankly for tourists, Kwun Tong itself kinda sucks. There really isn't anything to do there except look at rusting old factories and stuff like that.

As for Wan Chai, it isn't actually the premier nightlife district anymore. I think the main "hip" spot is probably Lan Kwai Fong, but that's the place to go if you want to see drunk gwailos (Cantonese equivalent of Gaijin, gwailo means "ghost dude") pissing in the streets at 3 AM. Wan Chai is more sedate, I don't think its as crazy as Roppongi (from what I read about Roppongi) but there's a lot of good restaurants and stuff there so its actually a pretty good place to hang out at night.

For street life, go to Kowloon. Temple Street Night market is good for visitors (Some locals hate it), just watch your belongings at all times. And just walk up and down Nathan Road and you'll see something you enjoy. There's something for everyone there.

And yes public transport in HK is second to none.