With optimism resurfacing over the global economy and the perception that major tail risks in the United States, the euro zone, and China have been mostly squared away, what's left to worry about?

We posed the question to our favorite analysts, economists, and traders across Wall Street – and we were surprised at the number of people who declined to give an answer, telling us that they just couldn't really think of anything too worrying right now.

However, we still got plenty of responses.

While general developments in the world's largest economies seem encouraging, there may be several reasons to be concerned lurking just beneath the surface.

Let's go to the charts.