Fondest memory: Most Thais do not speak nor understand English very well. This might be a cause for you to dislike to place. But that's easy to manage. For non-Thai speaking travellers, be sure to get a hold of a dictionary. If you are going to a place from your hotel, have the front desk personnel write out your itinerary for you in thai and english. Have them written in small strips (or you can cut them out yourself) and be ready to hand each to the driver when riding a cab. Try to memorize some words especially their numbers as you will need this for bargaining. If you have enough money, try to hire a guide.
Favorite thing: Thai is not really the easiest language in the world and almost impossible to speak for western people. So therefore, take a small Thai-English dictionary with you. Very few people in Thailand speak English and if you want to ask for information, it's very handy to have one. I took the Thai phrasebook from Lonely Planet. It's very handy, because the vocabulary is also written in Thai, so that you can actually show what you mean, instead of breaking your tongue trying to pronounce the Thai words ;-))