From Longman Dictionary of Contemporary Englishthe only thing/problem is ...the only thing/problem is ...spokenPROBLEM used when you are going to mention a problem or disadvantage I could take you. The only thing is Dan might need the car. → only
Examples from the Corpus
the only thing/problem is ...• In fact the only problem is deciding where to start.• It duplicates perfectly in production, so the only problem is making the first copy.• Well, no problem with the casting, the only problem is remembering the plot.• I did some um and the only problem is size.• But the only thing is they are really small. 