[@kapuchu] Ah, a fellow Sanderson fan! I think it might be hard to go back to Mistborn after reading the Stormlight Arhives, that's what I did, but the drop in quality was noticeable; he's a much better writer now than when he put out Mistborn (not to mention his earlier works). 

Anyhow, the magic systems in his settings follow a similar logic, they're all part of his Cosmere after all, so the same basic rules apply. That's the thing about him that I like - he follows the constraints of the system he's built, thus making even crazy feats of magic appear appropriate to the setting. I believe that's an important thing to keep in mind when creating such a system yourself - have a number of set rules that you can use as guidelines, so that you can know what characters in your setting can and can't do beforehand. This is especially important if you plan to use this in an RP, as players will always find a way to break your system :lol 

As for drawbacks, what I was thinking is that you could maybe play with the idea of Intent. People often have certain intentions, but when it comes to realising them, we typically get vastly different results than what we expected. So let's say a Sorcerer wants to conjure a bolt of lighting from his fingertips. Provided he has the required focus and strength of will, their Intent will transform into reality and some hapless bastard will get zapped. But what if they don't have the necessary willpower to do this? What if their Intent is too vague or maybe they [i]think[/i] they want to do one thing, but subconsciously [i]intend[/i] to do another? In that case, I'd say the Sorcerer might accidentally conjure a thunderstorm instead of a lightning bolt or summon an angry storm elemental, that sort of thing. 

Dunno if that makes sense, but it's basically the saying "be careful what you wish for, you just might get it". So all in all, not only does a Sorcerer have to be careful about their mental exhaustion, they also have to be very careful when considering just what their Intent is. Maybe that's what happens when magic "fizzles", you get something that you didn't quite intend. 

[@Shoryu Magami] The system you describe sounds intriguing, but as you yourself said, it's hard to discuss it in more depth without having access to more information about your setting. I will say, however, that you've been teasing us with your project in various threads and topics, so I hope that once you consider it to be ready you'll share (parts of) it with the rest of us :)