But that doesn't mean we're in a bubble this time around, says Mark Hulbert, editor of Hulbert Financial Digest.

"A sober assessment of the data shows that the market today is well less overheated than it was in December 1999, the first time this index traded above 4,000," he writes on MarketWatch.

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Hulbert cites several statistics, concerning the Standard & Poor's 500 Index, to make his case.

• The market's price-earnings (P-E) ratio, based on 12-month trailing earnings is 19.1, compared with 29.7 in December 1999.
• The Cyclically Adjusted P-E ratio, based on 10-year trailing earnings, stands at 24.4, compared with 44.2.
• The price-book ratio is 2.6, compared with 5.1 in 1999.
• The price-sales ratio is 1.6, compared with 2.4.

"To be sure, the bulls should not make too much of [those] comparisons, since the market in late 1999 was at one of its most overvalued states in U.S. stock market history," Hulbert writes.

But citing the Nasdaq Composite level isn't enough by itself to cry bubble now, he says.

Jonathan Golub, chief market strategist at RBC Capital Markets, certainly isn't crying bubble. 

"This year actually looks very normal in terms of a concentration of leadership" in the market, he told CNBC. "So there's nothing which looks like a bubble in this year. Could we get there? Maybe, but it's not a 2014 event. It'll be much further out."

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