Pronunciation.
For foreigners, one of the most difficult parts in learning pasa Thai is the pronunciation. The 44 letters, 26 vowels and 5 levels of intonation are the key factors attributing to those difficulties. Some words are pronounced the same but spelt and meant differently. Like the words which pronounced "mai" can be spelt in one way to mean "no" or "not" and in the other way to mean "burnt". So when you say "mai mai", it can be interpreted as "not burnt" or "much burnt" (Thai use repeating adjective to mean "much" or "more").

But, unluckily, that's not the point. The real problem lies in the fact that many Thai words are pronounced so closely that the foreigners cannot tell the differences. The word "glai" which pronounced with a bit longer sound means "far" while "glai" pronounced with a bit shorter sound means "near". So the way the foreigners will know which is which is the context of the conversation. How tough it is! But to Thai people, these words will confuse them more when spoken by the foreigners since they cannot pronounce them correctly.

A well known example for foreigners to practice Thai pronunciation is "Krai kha-ay khai gai" which means "Who sells egg?" Could you say it?