How hard is it to learn Thai?
Ummm, hard. The way I explain it is beginner Thai is very difficult, while beginner English is very easy. However, advanced Thai is easy, whereas advanced English is difficult.
Thai has 5 tones, I think 86 characters in the alphabet, 4 tone signs, and many of the words are very similar to each other, making it a pain. For example, you'll look up a new word and find out it's "leuy", but there's 500 words in the Thai language that sound similar to "leuy", so getting the pronunciation down can be daunting.
For another example, the simple word "mai" can mean anything from "no", a question particle, wood, new, or mile. The word "maa" can mean anything from dog, horse or "to come", and I'm not sure what the other two are. All depends what tone you use.
On the flip side, Thai doesn't have plurals, tenses, and so on. So once you get your head wrapped around the fundamentals, the rest is easy, as it's actually a pretty primitive language. Getting the fundamentals down is tough though.