[NOTE: We've moved this discussion from the thread at http://chowhound.chow.com/topics/826456 -- The Chowhound Team]
When I was taught how to make curry by a thai chef, I tried to cut down on sugar (as I don't like it either). It didn't taste very good and the chef explained that thai curry balances sour, spicy and sweet. Finding this perfect balance makes the flavors "pop". I don't believe you'd enjoy thai curry with no sugar but I do agree that some restaurants add too much sugar resulting in lack of balance.