"It's ridiculously complacent," said Evan Lucas, market strategist at IG, noting the price of protection, measured by the Vix, is at its lowest since 2005, suggesting the market is overly confident. "That's when retail clients start to jump on board. It's always at the end of a rally," he said.
Even the Federal Reserve has gotten in on the complacency concern.
Read More The risks of VIX-tied investing
"Volatility in the markets right now is unusually low," New York Federal Reserve Bank President William Dudley said last month, according to a Reuters report. "I am nervous that people are taking too much comfort in this low-volatility period and as a consequence of that, taking bigger risks."
To be sure, not everyone is on the complacency bandwagon.
"To say investors are 'complacent' is, in our minds, utter and total nonsense," Dan Greenhaus, chief strategist at BTIG, said in a note Thursday. "Saying others are 'complacent' is a shorter way of saying 'I've been concerned about X and thought Y would happen as a result and it didn't, so everyone else must be complacent,'" he added. "Because an outcome someone thought might happen didn't happen, doesn't mean others are complacent. It means they were correct."
Read MoreRecent market rally lacks conviction
He noted that conversations with clients indicate no shortage of worries, ranging widely from the Fed ending its asset purchases to high stock valuations to PC demand and Ukraine.
"There is much to worry about as there was in 2010, 2011, 2012 and 2013. And worry everyone did," he said.
Read More Low market volatility—a plus or storm ahead?