#1 is: Bill Clinton.
I'd say Obama should not trust Bill Clinton. Hillary has too much to gain from an Obama loss.
The other 4 are:
2. "Rebuilding 'You didn’t build that.'" (Especially with Elizabeth Warren as a prime-time speaker. She's "a political novice who will be making her debut on the national stage," and whose personal goal is winning in Massachusetts, and thus in being much more liberal than the average American.)
3. "Actual class warfare." (The Occupy protesters, who might have once seemed to be boosting the Democratic Party, are taking it to the streets of Charlotte. What if there's something 1968-ish?! And then there's just the danger of various convention-goers saying something non-mainstream.)
4. "Not a sell-out audience." (And with extra tickets being given away, there's an unusual danger of hecklers and protesters.)
5. "Can Charlotte cut it?" It's pretty small and ill-equipped for a city hosting an event this size.