OK, I will admit that I was being a little hasty with Soulcasting; it does have limits (currently it is directly related to the size of the gemstone, but more limits may arise). However, I will complain about AonDor.
I just feel like I have to chime in here that AonDor doesn't need to be "excused" for any reason. I thought it was a wonderfully written magic system, intuitive, consistent, and fitting for the world, with clear limits and consequences. Just one uninformed, ranting fan's point of view.
If you don't think it should be excused, I won't excuse it. I will instead complain about it.
Where are the "limits and consequences?" The system is, "if you draw the shape correctly, you get power." The only "limit" is distance from Elantris.
A magic system that is powerful isn't necessarily bad, but a system that has no limits is. It's nice knowing exactly what people can and cannot do. With the AonDor, Sanderson can do pretty much whatever he wants to if, say, Raoden draws the Aon "Eda." Since the AonDor is a language, it must have adjectives. Sanderson has never defined what the adjectives can do. In fact, he never defined what the words like "flower" can do. What happens if I draw the Aon "Opa"?
Another thing: the AonDor is a developing language. Maybe one day a character will draw a random symbol that will turn out to be an Aon. Then, Brandon has yet another Aon to do stuff with. He doesn't have to stick to the 41 Aons already made.
There are no power limits; it is implied that you can make it as powerful as you want with enough add-ons. They also never mentioned getting tired after repeated use of the AonDor.
Don't get me wrong: Elantris is a good book. But that is because there
isn't a magic system through most of the book. If Sanderson writes a sequel, he will have to put more limits on the system.