Further to your original post thebeargirl, the elevations in Thailand are very modest compared to anything in Yunnan. The highest mountain in Thailand is only as high as the western hills surrounding Kunming, so you won't need anything more than a light sweater.
The weather in Thailand in the dry season, covering all of northern, north eastern and central Thailand is superb. The chance of rain is almost nil, the visibility is superb and temperatures average around 28 degrees C in Chiang Mai, 31 in Bangkok and minimums are around 17 in Chiang Mai and 21 in Bangkok, although 25/33 even in December or January is not unknown in Bangkok. Basically, it's hot, sunny and less humid and cooler than other times of the year but that's it.
I doubt that the water quality in Thailand could be any worse than China. I would never drink tap water in China because that would make one very sick, and I would advise the same in Thailand. If you have a weak stomach, brush your teeth with bottled water only.
Thailand is generally safe, but as a freer country than China with less police control and far more tourists and expats, you'll find things a bit different especially in nightlife areas. While by and large safe, don't attract the wrong people or you could get in trouble.
As soon as you arrive in Bangkok, you'll see people from India, Europe, Africa and other places, whereas in Kunming you might see 3 foreigners roaming downtown, in Bangkok half of all people in the central shopping areas will be foreigners. The difference between Kunming and Bangkok in this respect is like night and day.
Finally, never ever lose your cool in Thailand. Nobody will help you or talk to you again if you do. Getting visibly angry might work in China, but it gets you nowhere in Thailand. You'll almost never see Thais get visibly angry, so I would advise you to heed this crucial piece of advice.