Worth saying that Oden is an unusual case as a rookie because of his young age, the quality of his teammates and his recovery from injury. It muddies comparisons to nearly all other big men going #1 in the draft (Dwight is probably the closest). The only way I see Greg Oden disappointing is if he gets injured. He's too good not to contribute a great amount to his team.
As for my expectations of the man, stats wise I expect Oden to put up 12-15ppg, 9-12rpg, 1.5-2.5bpg, 31-35mpg.
Performance wise I expect Oden to be the leading factor in turning Portland into a quiet but potent contender. They'll likely be a lower playoff seed but they'll be extremely dangerous. They're going to cause a lot of those teams out West a lot of problems.
Oden's biggest impact on the court will be enabling his teammates to do what they do best, Oden will do all the dirty work and take up a lot of defensive attention from the opposition (too big, too tough to stop around the rim, like Shaq these days, he'll always have a defender glued to him pushing away from the paint). That'll allow his teammates far more freedom to go about their business, I expect Aldridge to be the biggest benefactor.
I expect Oden to be a Top 6-8 center by late January. I reckon he'll have enough offense to separate him from the Dalembert/Chandler group of centers. Depending on how involved he is offensively he could move up to Top 5 by the end of the season. I expect him to be one of the best interior defenders and rebounders in the league.