Lesnar did not impress the Vikings coaches at all -- they were talking about it just yesterday on Sirius. He's a crazy physical specimen, but he never played football before and couldn't get a hang of even the basics apparently. Football technique, ability to get down low, etc... he was light years behind the curve according to those who saw him in camp. Practice squad guys were pushing him back.
And think about this: at his current size, he wouldn't even be all that big of a NFL defensive linemen, which shows you how nuts the NFL game is today.