Fun thread discussion.
The speed of the game would eat up larger athletes, especially today with how the game is played. Chara's actually a good example, his stamina is legendary and he's an insane athlete but even at his lighter (relative) weight he can barely keep up with the speed of today's game. Someone like Watt, given elite skating ability, would be a Lindros-esque dominant player for sure but he'd be a bit of a lumbering mass on the ice compared to the smaller players. Now, he'd be able to bowl over guys on the ice and I'd love to see him lay into someone in open ice, but I'd imagine he'd have a tough time controlling the puck. I think of the best stick handlers and they tend to be shorter, quicker guys, a huge 6'9" Watt type player just wouldn't be good with the puck.
As a stay at home defenseman? I think he'd be real effective, and he'd be the ultimate enforcer type. But I don't think he'd make a good forward, and goalie is out just because it's all about quick-twitch response and flexibility, more muscle mass does absolutely nothing for that position.