Gee, you know, I really hope that Obama will win. I don't even think that the differences between Obama and Clinton are irreconcilable. They're pretty close when it comes to the actual issues, despite my personal views still being more aligned to Obama's. The problem, however, is that if Hillary Clinton does receive the Democratic nomination, then all of the independents and floating voters that Obama has been attracting are going to go to McCain. McCain has shown himself to be particularly good at attracting votes from this sizable group, whereas Clinton is terrible at it. Sure, she'll get some of them to support her, especially if Obama says "sorry guys, I lost, but please vote for Hillary instead", but she is simply not going to be able to retain them all. They don't like her.
Another thing that's extremely important is that if Clinton gets the nomination, then all of the Republicans who would otherwise stay home or support McCain from a distance, are going to run to the voting booth to vote against Hillary Clinton. They don't like her.
A Clinton nomination will seriously endanger the chance of a Democratic victory. That's the worst that can happen after eight years of Republican dominance.
I really don't have a problem with Clinton being the president, but I simply don't think she'll be able to reach that point. Obama has a much better chance, and that's why he needs to win this nomination so desperately.