From my understanding of an offensive playbook...
In a true spread attack, a QB will typically make 2 reads, at the most, unless he's certain he has more time to make that 3rd or 4th read. After those 2, if nothing is open and if he feels pressure, then he either takes off running, or dumps to the safety valve if he's still trying to gain positive yards. Otherwise, he'll simply throw it away.
It's supposed to be an easy, simple system that can be rapidly implemented at the high school or collegiate level, and that it can help equalize things when it comes to a difference in athleticism.
The Brian Kelly / Tommy Rees offense is more of a hybrid. The base is still the spread, but it also had some pro-style elements in it that closely resembles the K-Gun offense that Jim Kelly QB'ed in Buffalo during their Super Bowl runs.
In a nutshell, this offense focuses mostly on the short passing game to set up the run and the downfield passing, but it's definitely not a simple offense.
From what others have told me, the playbook is very hefty, and more complex than the scaled back Erhardt-Perkins playbook that Charlie Weis used here (which worked very well).
I understand that complex offenses can be quite effective, if you have a veteran group who has the experience. This is why full-fledged pro-style offenses in the NFL work very well, because these guys have been at it for years, practicing with the same guys for a while, etc.
At the college level, you don't have that kind of luxury for the most part. You're going to be lucky to get 2-3 years of starting time out of your players, and the team chemistry doesn't have enough time to develop for that kind of complexity.