I don't think anyone thinks Greenway is actually at the same level or ever will be at peak Staal.
Eric is big, deceptive in his skating, really good with his stick, and is a very accurate shooter. Underrated shooter across the league. Guys know he can shoot really well but they forget. He's also super smart. At one point Staal was probably a top 10 player in the league (05-08 area) or close to it. That also means the world. Obviously he also is good defensively too, though his +/- doesn't show it while playing for Carolina.
Greenway will be a success for us Wild fans if he can put up 60 points and 20 goals a year while playing 'like' Staal.
The thing I see in Greenway's game is that he is tenacious and actually a bit deceptive in his smarts. He skates well and makes quick decisions with the puck. He looks confident in his body language on the ice and doesn't shy away from contact even if he isn't overly physical.