McDonald was a Division II prospect from Grand Valley State University (GVSU), so he hadn't played against top competition when he was signed by Green Bay as a UDFA in 2010. He has nice size at 6'4" 310#. He failed Patchick's "vertical" test, with a vertical of 27" prior to the 2010 draft, so he may not have been quite as explosive as a typical Pats interior lineman.
During his rookie year with Green Bay, Mike McCarthy had this to say about McDonald:
McCarthy high on rookie offensive lineman McDonald - JSOnline
As I said above, McDonald was cut in a numbers crunch similar to what happened to Ted Larsen with the Pats, not because Green Bay wasn't high on him. As far as I can tell, his main "weakness" is his small school background and lack of experience. His main strengths seem to be toughness, work ethic, agility and smarts.
Tom Brady, Matt Light and Brian Waters both some interesting comments when Waters started at center against Indianapolis. Waters helped McDonald with the silent snap count because of his lack of experience. Light commented:
Nick McDonald Receives Help From Brian Waters During Lesser-Used Tactic on Silent Snap Count - New England Patriots - NESN.com
Waters lauded McDonald's preparation:
Brady noted:
It Is What It Is » Nick McDonald happy to get his first NFL start under his belt
It sounds from all of this like McDonald is a smart kid and a really quick study. He had never played center until signed by Green Bay, but was good enough to be discussed as a possible successor to Scott Wells his rookie season. And he managed to learn the Pats' system his first season well enough to step in and play productively in 2 games as a starter. Combine that with his size and reported "quick feet", and he sounds like a guy with a lot of upside as a possible future starting center.