Having lived here 6 months now, I've learned a few things that might help you out if you decide to come to Thailand. Or you know, it might just help you decide *not* to come to Thailand. As the Thais say "Up to you!"
It's hot in Thailand. It's humid in Thailand. It is often both hot and humid in Thailand. And then there's the rainy season. You will sweat, accept it.
If you're coming for a vacation, it's not much of a problem, just wear light loose clothing and enjoy yourself. If you're coming to live, and possibly to work here, talcum powder is your new best friend. There is nearly an entire aisle in Tesco devoted to different kinds of talcum powder. Johnson & Johnson is represented of course, as is Care and Baby Mild. There are cooling powders (which I didn't actually enjoy), and even liquid powders. Douse yourself liberally after you shower and you will feel comfortably dry for as much as 10 minutes before you start sweating again. =)
The currency is the Baht (about 34 baht to the dollar at the moment), it's all different sizes and colors. The coins descend in size as they descend in value. It all has Arabic numbers as well as Thai numbers. Spending money is one of the easier things to do here. ;)
Learn to say "hello" (sawatdee kaaa/kap), "thank you" (kop kuhn kaaa/kap), "how much?" (tow rai?), and the numbers before you come. That'll get you pretty far actually. Don't worry about the wai (the traditional Thai greeting) - you're not expected to understand it, and frankly you'll look a bit silly if you do it wrong (one does not wai to someone of lower status - for example waiters/cashiers). Smile a lot.
If you are staying in a touristy area such as Bangkok, Chiang Mai, Phuket, or Pattaya, you'll be in food heaven. There will be plenty of western food, and a lot of the Thai restaurants will have menus in English or at least picture menus. If you go further a field, you'll want to learn a little food vocabulary, rice, noodles, pork, chicken, vegetables etc. If you are vegetarian or vegan - good luck, nearly everything has fish sauce in it.
The food here can be very spicy. It’s traditional to put 7 chili peppers in a dish of Som Tam (papaya salad)... and we're not talking about jalapenos or serranos here... these are the itty bitty nuclear peppers. And quite frankly 7 of any kind of pepper is overkill in my opinion. But I am not a fan of mouth burn. Spicy = "pet", a little spicy = "pet nit-noy", not spicy = "mai pet"... and if you are extraordinarily insane / courageous; Very spicy = "pet mahk mahk"
Unless you have intestines of steel, or only eat at fastidiously clean indoor restaurants (no fun!), you will probably get a stomach bug. Bring chewable Pepto-Bismol tablets with you and keep them handy. If you've been sick for more than 3 days, IMO that's when you bring out the antibiotics.
Tuk tuk drivers in Bangkok will try to scam you (there is no free-gas day). In fact, EVERYONE in Bangkok will try to scam you. Be wary of any Thai person just hanging out by a tourist site, especially if they speak English really well. If you are lost or in need of help, find someone who is working in a restaurant or a 7/11 and ask them. Even if you do get scammed, try not to let it ruin your trip. You’re only out a little money and now you’ve got a good story to tell.