East Asia is the most populated area on earth, and thus there are many varying regional cuisines. However,
Chinese, Korean, Thai, and Japanese food do have a lot in common. In the East Asian culture, food, spices, and seasonings transcend taste. Food is an art, a medicine to promote healthy life, and even a "symbolic offering to worship ancestors." As for ingredients, they all have developed to emphasize oils, fats, and sauces within their dishes. The three most common cooking methods are sir frying, steaming, and deep frying. Common ingredients include soy beans, mung beans, seafood (especially in Japan), tofu, ginger, garlic, sesame seeds, and tea. Of course rice is also a staple ingredient of East Asian cuisine; however, the type of rice varies from country to country. Long-grain rice is popular in China, Jasmine rice is popular in Thailand, and in Japan and Korea, short-grain rice is popular. However, the cuisines of these nearby countries often mingle and influence each other; to the average non native, it is difficult to distinguish between Chinese and Japanese food, or between Thai and Korea. Yet, by making slight generalities, clear cut culinary differences are present.
Noodles are a main culinary ingredient in East Asia and tend to differ based upon which region one is in. Thailand and parts of China tend to use noodles made from rice flour, whereas Korea, Japan, and most of China tend to use alkaline wheat noodles. Soba noodles are very popular in Japan, and sweet potato starch noodles are very popular in Korea. Moreover, Thailand is known for its fish sauce and peanut oil, while Korea, Japan, and China mostly use sesame oil and soy sauce.
Japan and China are different than Thailand and Korea, in that they routinely use alcohol in their dishes. Some American Thai restaurants marinade meat in whiskey, however this is not common practice is Thailand. As for which cuisine in East Asia is the most spicy, Riam Chantree (the most viewed writer in Thai Food on Quora.com) put Thai food as the most spicy, followed by Chinese, then Korean, and lastly Japanese. Japanese cuisine is known for thick, sweet sauces - one example being the very popular Japanese version of worcestershire sauce (Britain origin).
Thai food has roots in Chinese cuisine, however there are significant differences between the two cuisines. Thai food does not use as much ginger as Chinese food, especially in stir fries. Chinese stir fries are typically done at a higher temperature than Thai stir fries. Furthermore, coconut milk is much less prevalent in Chinese dishes than in Thai dishes. The cuisines of East Asia are so popular that they have spread all over the world. Chinese, Japanese, Thai, and Korean restaurants thrive in the Western World (although sadly, to us fans of traditional East Asian cuisine, the far east food is often "westernized" by European and American chain restaurants).