Well no, it certainly makes a incremental difference in the quality of their play and later on can play a big part in their relative career longevity, but no, work ethic cannot make a player great. The closest thing to an NHLer who made it on work ethic alone was Mike Keane.
Conversely, a supreme lack of drive or intensity can completely derail an otherwise talented player. That's why I still look at Wojtek Wolski with such frustration. Dude had game-breaking, elite talent. He could take over games when he wanted to...alas, he just didn't care enough.
Brett Hull might be the ultimate example of a guy who was overlooked initially and then excelled despite a less-than-stellar work ethic.
Still, I think it's nice to see someone truly want it enough to hone their craft (and their own fitness level) to the best of their ability. When they have the talent AND the work ethic, those are the ones you keep as long as possible.