Another potential factor in a second-half slowdown: Europe's spotty recovery. "The Euro-zone endured a deep recession, and recovery has been sluggish," he noted. That, of course, presents a mixed bag for the import-export business because, as he explained, with the US growing twice as fast as Europe, this country is now importing more, lifting Europe's exports. The flip side is that Europe, with its ongoing fiscal woes, is importing less. "Weaker exports to Europe could stunt US payroll growth," Vitner explained.

At the same time, he pointed to several European countries that will find it difficult to "grow their way out of their fiscal problems," he said. In particular, countries like Ireland, Greece, Spain and Portugal "need to undertake a sizeable fiscal adjustment to stabilize their debt-to-GDP ratios." Vitner conceded, as well, that to a certain extent the US faces the same problem, "which is not comforting." Another major risk from the sovereign debt crisis is that so much of the banking community is international in scope, which links all of these national crises together so closely.

For commercial real estate, the result has been not a lot of transactions, but those that do occur happen "at very attractive prices," Vitner said. "Commercial real estate represents a downside risk to the US economic outlook. The worst of the decline is over, but the problem persists."

Lower interest rates will be a plus. And with so much risk and higher prices in Europe, "direct investment in US real estate is looking much safer by comparison," Vitner said. In the final analysis, "growth in global GDP will be negative this year for the first time in decades," he concluded.