Sweet, salty, bitter, sour and spicy. A good Thai dish should have them all.
The people of Thailand LOVE to eat, and even more so, they love flavors. The smallest ingredients are cherished to enhance the taste and enjoy-ability of each piece you put in your mouth. Bites are built individually to include a bit of each of the foods on your plate, and the extra-perfect bite is made for your partner.
There is sugar. After the initial excitement of trying and relishing in the pleasure of every Thai dish I order, I have realized that sugar goes into almost all of these dishes. Sweet is an important component to the flavor, and many people from abroad are not used to tasting savory dishes with such sweetness. A spoonful of sugar is usually thrown in, and yes it makes the dish delicious, but I can certainly taste the sugar in most dishes now, and frankly, I prefer it without. But this is not about me. I just wanted to inform you that people LOVE sweetness, and you will quickly notice that everything is sugared up. It shows in the coffee, which always comes sweetened. Your tea? Your milk? Your fruit? Your shakes? Your noodle soup? Likely all sweetened, unless you ask for it without. Use "mai wan" to ask for something to be made unsweetened.
On the contrary, oh the sour! Many fruits are consumed before they are ripe, because sour = delicious. Especially mangoes. Green mango, eaten with sugar, salt and chili, is an absolute explosion of flavor, and one of the most enjoyable snacks in the land.
But is Thai food really spicy? Um, yes. It will probably blow your face off. If you are thinking you are the master of heat and want to test this theory, go straight for a som tum (papaya salad) and order however many chilies you want with your salad. Try them all, but work your way up. You can order from 1-6 chilies in your som tam, and trust me, 1 chili is spicy. Actually, zero chilis will still be spicy, because they make your salad in the same pok pok they made the last salad in. Your salad chef will certainly add more if you ask, but you might get a reaction and some giggles for thinking you can handle the heat.
Most food here comes with a little heat, and the majority of dishes have straight up chili as an ingredient. Locals generally don't believe foreigners can handle, or even like spicy food, so you might get asked if you want spicy, or even served a dish made especially mild for you without even having asked for it. If it's for you, tell them spicy is ok ("gin pet dai!"), and if you're a heat chaser, ask for Thai spicy ("pet Thai"). They will make it like they would for a local.
Want to order food? Here's your reference guide for some amazing dishes commonly found at restaurants.
Krapow moo krob - Crispy pork with basil
Krapow gai - Chicken with basil
Kana moo krob - Crispy pork with kale
Gai pad pet manuang - Cashew chicken
Pad prik gaeng gai - Chicken with chili paste
Khao pad - Fried rice
Rad na - Rice noodles in gravy
Gaeng hinlay - Northern hinlay curry
Gaeng kiao wan - Green curry
Tom yum goong - Sour spicy soup
Tom ka gai - Coconut chicken soup
Moo nam tok - Grilled pork salad with toasted rice
Woon sen - Glass noodle salad