I think if you were to ask most people if they like Thai food the answer would likely be "yes." Thai food is, without question, the most popular South-East Asian cuisine in the world and one of the most iconic types of food overall. However, it surprises me to find out how very few of my western friends who love Thai food, are completely unfamiliar with a very major part of it - Isaan food.
Isaan (sometimes spelled Isan) is pronounced "eee-saan" and is a major part of the country. It consists of twenty provinces in the North East of the country and comprises nearly a third of the landmass of Thailand.
Most people who travel to visit Thailand do not go up there and to be honest there are lots of reasons why that is the case. There isn't much to see up there. The area consists almost entirely of flat farmland and the scenery could be considered to be quite featureless outside of the northern, mountainous areas.
Now another (quite tragic but true) aspect of Isaan is that for the most part it is dreadfully poor. One third of the entire Thai population lives in Isaan but they manage to acquire only 10% of the overall wealth. I think that this lack of income contributed to the citizens becoming very creative with their cuisine and over the eons we ended up with Isaan Food, which is remarkable and you should definitely try it.
Isaan food encompasses a lot of different dishes, but the most famous one is "Som Tum" which is a salad made of shaved papaya (most of the time) or mango. This is put into a giant bowl along with citrus, sugar, red onion, tomato peanuts, and chilis (and a wide variety of other optional things) and then bashed with a giant pestle until all the flavors are mixed together.
It is normally eaten with sticky rice which is used to soak up all the juices that were released during the "smashing process."
This can end up being blisteringly spicy so be careful. I can handle quite spicy food but if i put more than say 5 chilies in mine i wont be able to eat it. I think 1 or 2 chilies is a good way to start for the uninitiated. One time some Thai friends of mine were eating a somtum with more than 20 chilies in it and they invited me to share. I took one bite and spent the next hour with hiccups and drinking water to try to put out the 5-alarm fire on my tongue.
Another staple is called "larb" which is a minced meat salad that can be made from just about anything you can imagine. I have seen one that was made with grub worms and while it tasted fine, I am not a big fan of eating worms so I would say stick with chicken, pork, beef, or fish. Again, this one can end up being very spicy so be sure to ask for not spicy (say "My pet" for Thai) and take it from there.
Roasted meats or sausages are normally served with this as well. My favorite is the roasted pork but again all the other meats are available as well. They are served with different sauces and I think it is best to just take the chef's word for it on which one to use. Whole BBQ fish is a common site here as well.
I think the best part about going to an Isaan restaurant is that it is expected and customary to order for the whole table. Each individual dish is quite cheap so ordering say 10 things for the everyone to share is the name of the game. If one of the dishes ends up being the favorite don't worry, you can always order more. It is expected of you.
There will also be a lot of fresh vegetables served with the meal including lots of fresh herbs. Some of which I don't even know what they are but i recommend trying them all because you never know when you will stumble upon something amazing and unexpected.
While I would imagine it is quite rare to find a restaurant abroad that sells ONLY Isaan food you can probably find a dish or two on the menu of your local Thai restaurant - more likely than not it will be "Som Tum." It should come as no surprise that these places are going to be better in Thailand, and much much cheaper. Isaan restaurants are not just in Isaan, they are everywhere. You could be cruising down the side of some random-ass road in Thailand and there will be an Isaan restaurant. I once lived in a jungle community of just a few hundred people for miles in any direction and we had two of them.
So if you find yourself in Thailand try your hand at this very famous style of Thai food. If you can manage to not like any of it i would be genuinely surprised and even if you don't, it is some of the cheapest food in the land so at least you had an experience! Step away from the Pad-Thai and friend rice and have a look at some of the most authentic but rarely spoken-of Thai food in existence.