I see how old this thread is so I'm sorry that I'm so late in joining the debate. As a young person I naturally fall into the 'thing' side of the argument given, as many have already explained, its rise in popularity in the 80s. Although I can understand the historical logic of 'think' I do believe there is absolute sense in the se of 'thing' this word represents an undefined abstract which fits perfectly into most sentences of this phrase. I find 'think' too limiting as it is not necessarily that the person will have to mull over or reconsider their belief or opinion. Rather it could be that there will be repurcussions for having this opinion or standing, eg 'if you think buying that dress is a good idea you've got another thing coming'. Of course this 'thing' could be a 'think', a reflection on your action with regret, or this 'thing' could be more substantial; perhaps ridicule by your peers or a huge credit card bill. I can understand the value of 'think' but find 'thing' all together more useable.