Seoul doesnt really have any amazing sights for a tourist, some nice places to walk around, yes, but nothing world-class. Some people hate it, some people love it. The food can take a bit getting used to, and if you are eating at local restaurants, you will have a hard time finding an english menu. Prices will be much higher than in Thailand (esp for your hotels). It really depends on the cost of the stopover for me, if its free, on a neglible amount, I would go (if you have never been), but if its going to cost you quite a bit more, I wouldn't bother.
Also, Incheon airport is over an hour away from the city, so think of that if you are planning on stopping for less than 24 hours.
I do love this city though.