The only thing is…
Definition 1 of 1
Pro Tip 1/3Hesitation
Signals a pause before mentioning a drawback or concern, usually in a casual conversation. 
I'm excited about the new project. The only thing is, it might overrun our budget.
Pro Tip 2/3Softening Effect
Used to gently introduce a negative point or criticism, making it sound less harsh. 
Your design looks great. The only thing is, the colors don't match our theme.
Pro Tip 3/3Conversation Flow
Helps maintain the flow of conversation by acknowledging what was said before introducing something different. 
Sure, we can leave at noon. The only thing is, we might hit traffic.