According to the 2007 economic review of the Bangkok Post, less than one in six of Thailand's labour force paid income tax. 10% of these taxpayers paid 90% of the income tax.
So it's not surprising that Thai politics is dominated by various kinds of 'populism' - that is, policies dominated by pork barrel programmes. The wonder of it all is that Thailand is running both a small fiscal surplus and a fairly large current account surplus.
Room to expand the tax base is limited, however. 49% of the population works in argiculture, producing just 10% of GDP. Average monthly income in agriculture is around £60 a month, so there's little to tax. Incomes in financial intermediation and education pay six times as much, and the multinationals over twenty.