For Canada, I don't see there being one impact player right now. Carter, Getzlaf, Stewart and Burns are all big guys who can dominate physically and offensively. But at the same time, you have Tambellini, Richards, Crosby, Dawes and so on and so forth that are smaller guys. Canada unlike in some years past, doesn't have a go-to guy on offence. That being said, I bet one of Carter, Getzlaf or Burns (the #1 line probably) will lead the team in scoring. Obviously, Fleury is Canada's impact player. Coburn is the top defenceman, but won't be considered an impact player unless he contributes offensively, which is not his style.
I think O'Sullivan, Suter and Eaves will lead the American squad, Barulin along with Ovechkin and Malkin for the Russian team. Toivenen is the key for Finland, and Steen and Nilsson are huge for Sweden. Hudler and Fleischman are the top players for the Czechs, although as for Schwarz, I'm not sure he'll get the nod over Mensator. Vrana is huge for the Slovaks and Ramholt, Bartschi and Romy are the keys for Switzerland.