While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of FireEye Inc (NASDAQ:FEYE) plunged 7% today after Barclays downgraded the enterprise-software company from overweight to equal weight.

So what: Along with the downgrade, analyst Raimo Lenschow planted a price target of $73 on the stock, pretty much in line with Thursday's close. While Lenschnow thinks FireEye's cyber security technology is truly differentiated, he's worried that the seemingly stretched valuation might limit the stock's upside in 2014.

Now what: Barclays remains bullish on FireEye's longer-term-growth prospects. "Technology is still disruptive: FireEye is changing the way customers approach cyber security and disrupting legacy vendors who still largely rely on outdated, signature-based approaches," noted Lenschow. "As we have previously shared, the acquisition of Mandiant expanded the company's TAM by introducing key market adjacencies like endpoint, incident response services, cloud SIEM and managed defense." With the stock trading at a whopping price to sales of 65, however, it's easy to understand Barclays' nervousness about buying into those prospects right now. 

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.