Not sure how you measure development, but things like the level of industrialization, income per capita, gross domestic product per capita, life expectancy, the rate of literacy and so on.
By these measures, Thailand is far from being a developed country.
One major problem for me is that I don't trust the government to tell the truth. I remember the case 20 odd years ago when the government announced a literacy rate of 95%, but friends at the Ministry of Education told me it was actually 65%. The government was too embarrassed to reveal the real number. Losing face makes them try to look good.