One common question that usually pop up into your mind and you want to know the answer while planning a trip is when is the best time to visit that place. If you ask me the best to visit Bangkok, I would say several times. And, New Year is one of them because of these reasons.
Thailand is a tropical country which has hot and humid weather for most of the year. The cool season is between November to February (sometimes shorter). During this period, the weather is perfect for exploring the city, not too hot and not too cold. In Bangkok, our cold season is about 25 degrees or so. Usually there is no need to wear jacket.
Nice Street lights
Even if majority of Thai are Buddhist, during Christmas and New Year the shopping malls and hotels are decorated beautifully especially around Rachaprasong area. See the street lights last year here.
Shopping malls and temples are open as usual
Shopping malls and temples in Bangkok are open daily as usual. Only some shops outside the shopping malls close.
Less traffic/ less crowded
During New Year, you will see very less traffic and crowded in everywhere in Bangkok because many people, who are working in Bangkok, go back to their home towns to visit their relatives. Besides, many people, who are living in Bangkok, go travelling around Thailand because it is a long holiday (around 5-9 days). Frankly, I really love Bangkok during New Year.
So, what about visiting other provinces in Thailand?
It is a great time to visit Bangkok. But, if you want to visit other provinces in Thailand during this period, there is thing you need to be aware of. There will be many people going out of Bangkok and the traffic is jammed. Usually the bus is full; you will get a ticket only if you book it in advance. My recommendation is either go early (go before the first day of holiday) or go late (go after the first day of holiday). Try not to go on the same day with other people.