Scoring was way higher in Mario's rookie year. His 100 points were good for 12th or 13th in the league, while Crosby will end up 8th or 9th and he may not even score 100 points, to put things in perspective. Wayne Gretzky had over 200 points that year.
Something you don't consider is the impact that improved goaltending has had on the game. Look at highlights of Mario's early career. The typical goalie just wasn't that good compared to today.
Just on a point production level, I think you can argue that Crosby's having a more impressive rookie year than Lemieux. In fact, I think you can argue that Crosby's having the best season ever by an 18-year-old rookie. But Lemieux came into a team that had nothing and still put up huge numbers. Crosby's on the worst team in the league, and it's not as if he's been playing with superstars all year, but playing with the likes of Palffy, Recchi, Gonchar, and even Armstrong certainly helps him a lot.
It's just hard to compare because these are two entirely different eras and two different teams, even though they both were very bad. The only thing we can say with certainty is that Crosby's having about as good a year as anybody could have expected. He's having one of the best years ever by a rookie, especially one so young.