How much better could this movie have been if...

Justin Long would have been entertaining...but Shia LaBoeuf? Really? The guy who irritated his way through Transformers and contributed to the mess that was Crystal Skull? Nay, was one of the main problems in the film.

I think Jay Baruchel fitted the role, but I don't think he played the role very well at all. I think the problem was his age, as he was too old for the role. Remember, he was meant to be 20, not a guy pushing 30 (he was 28 when he made the film). In this way, he sort of lacked the necessary knowledge to play the geeky, awkward kid, or at least he was unable to tap into the role well enough.
He played the part like an adult.
There were parts of the film I liked, I mean, Nicolas Cage was more restrained than normal, and I preferred it far more to Percy JAckson (was bored throughout that entire movie. Sorry.). The main problem however, was that the villains were not threatening or powerful enough (Molina went for comedy, as directed, far too much. He played a better villain as Doctor Octopus.) and we never get the threat of danger from Morgana. The villains are just not powerful enough, because they get swatted like flies far too easily when they appear. Morgana's just not introduced well enough to create danger. We only have 5 minutes of Morgana killing a Merlin who has been introduced for 5 minutes also. So no drama there. No threat of her revivial either.

The story needed work before ever getting to the development stages.

But it was still fun. IT passed an afternoon for me. I don't think it should have taken such a hammering at the box office however. Worse films have done far better.