Thai people. Smiling polite happy and friendly people. Hard to get any better than that. Get away from the main tourist areas and meet normal Thai people and you'll be in for a treat.
The natural environment - there isn't much of it left, but if you can find some tropical jungle, untouched beach, limestone caves and cliffs - very beautiful.
Thai food - there is plenty of this and it is great.
Rich cultures and history - The T'ai people are the biggest ethnic group in S.E.Asia and it is believed their history goes a long way back. As you start to learn about Thai culture and it's intricacies, this is easy to believe. But there is also an amazing richness in ethnic and cultural diversity in Thailand. From Muslim Malays in the South to Khmers, Chinese, recently nomadic tribes, the left overs of the great Mon civilisation, and to the numerous Hill Tribes in the North, and much more - Thailand is a rich and diverse country.
The love of the Thai King and the love from the Thai King. For the last 60+ years Thailand has been very lucky to have a great leader and King who oversees the country with wisdom, patience, love and compassion for the people and environment. The Thai people have a huge respect and love for their King and to witness this is a special thing also, in a world that otherwise seems obsessed with money, power, greed, etc., it seems well worth seeing an alternative.
Compassion - again following on from the previous points - it is great again to get away from the tourist areas and see so often in normal Thai life, people are very giving and caring towards each other and others around them. Get up early to see people giving alms to monks or head into the parks and watch people going out of their way to give food to stray dogs, or to feed the fish, etc. Nice.