Nieuwenhuis is big, strong and runs well. He looked plenty rangy in centerfield in spring training. I suspect he won't hit for a high average immediately, but he will draw walks and hit for power. As a 23-year old in AAA in 2011, he hit .298/.403/.505 with walks in 14.5% of his plate appearances. Nieuwenhuis has improved as he's moved through the system too: in 2010, he combined to hit .274/.327/.475.
The minor league equivalency calculator puts Nieuwenhuis at .251/.339/.411 based on a translation from his 2011 line. That looks about right to me.
Moving back further, and I referenced this on the forth-coming Mostly Mets Podcast, Nieuwenhuis took a dramatic step forward in the 2009 season when he learned to turn on inside pitches and pull the baseball. He hit .345/.406/.647 in August 2009 in advanced-A and has never stopped hitting.