I think those that criticize Obama for it are being quite petty when they themselves have not done it nor are doing it now.
It's more internet bravado IMO than anything else. And that includes those that criticized Bush's speeches as well. While gaffes are funny, they are hard to prevent when someone is a public speaker. I don't think there is ANYONE (especially in this internet 24-hour news era) that has not made a Gaffe or mistake.