The big thing is this... an executive order cannot create a criminal offense. Obama cannot suddenly create a new crime by means of such an order. So, for a completely made up example, let's say he issues an order which bans the civilian possession of an "assault weapon". Makes it a federal felony offense to be in possession of an AR, M4gery, AK, etc. Has this enforced by federal LEOs (and state level, who can arrest for federal offenses. Whether the do so or not is a different issue). Defense lawyer will move to dismiss on the grounds that POTUS acted ultra vires (I think that's the Latin phrase), meaning he went beyond his Constitutional powers by creating a law, a power which is specifically enumerated to Congress. Case dismissed, and law ruled unconstitutional.
Now, can he issue an order which changes the meaning of terms in existing laws, like expanding the definition of dealer to include anyone who sells a firearm? Again, no. The power to change the laws remains with Congress.
What if the term is vaguem and they interpret the term in a new way? Now you're in the grey area, where lawyers and courts will debate the legality and constitutionality of the order for years.
If I had to guess, I'd wager on the no-fly list, or maybe restrictions on imports. But again, it's a pure guess. The first might get struck down by a court. The second would probably survive a challenge.