Ok, Clark's now officially in.
The big question everybody is asking is "how can a man with no political experience become president?" That question is somewhat misleading, however, as the answer is "he wins the most electoral votes".
The real question is "can he successfully campaign for the presidency"? Many are asking whether his inexperience at politics will lead him to pull a series of unintended gaffes that a more experienced politician would be able to avoid. Honestly, though, I'm skeptical as to whether this will actually happen. It's undoubtably true that Clark made this decision a long time ago; whether he was pulled into it by the "draft Clark" movement or not, he will have been preparing for months, and perhaps even longer. He's also being advised by people who have been involved in politics all their lives, and I doubt a former general would have problems knowing how to handle advisors.
Even if he wasn't a politician prior to this run, he's been in training to become a politician for a while now. This run has been thoroughly planned and gamed out, and this announcement is in a way the end of this training, rather than the beginning.