Two things : first, I agree with what Tumblezweedz said. National New Years, special Festival from other calendar, as in China, or Nepal, for instance make great cultural experiences. I lived in few in Nepal. In one, children throw balloons filled with coloured water at people on the street, during the whole day ! They hide on rooftops, or in bushes and bang/ploutch ! This festival has become a "distortion" of an ancient religious fest. The elderly aren't happy about the way it turned out, how crazy and unreligious it is nowadays.
Here's couple of links :
New Year's customs around the world
goasia : new years of Asia
Secondly, Christmas and New Years, western ways, are a blast in Thailand and Hanoi, from my own experience. For Christmas, I was in Hanoi. The young Vietnamese, buddhist of course, took control of the streets at night, it was crazy ! And lots wear the red Santa Claus hat, hilarious ! There is one beautiful catholic church, the only place to celebrate religiously Christmas, if you want. With other travellers, we went to a chic restaurant, and the whole meal for four costed... $30 total. We then went out in a «non-backpacker» club, it was a bit quiet, but at least vietnamese
(sitting on pillows on the floor, having to take our shoes off, one foot-high tables, etc.). We enjoyed it very much, especially the litchi alcohol ! There were bars loaded with partying travellers, if this is what you're looking for.
In Bangkok, I left the morning of the 31st, so I wasn't there for the big night, but they were building stages for live music. There was a spectacular crowd of Thais and travellers in the streets, electrical ambiance, and that was only daytime !
In Nepal, tourists gathered in clubs, that's all. I arrived in Kathmandu on the 31st, at night, and nothing was going on in the streets, nor in my hotel as a matter of fact. So I slept.