The thing I like most about Frank is that I think that he didn't take himself too seriously as a musician, but he took music VERY seriously.
That's a good observation. There is nothing gratuitous about his work. He wasn't trying to "impress" others with his complexity or virtuosity, his music merely followed the ebb of his artistic vision. Sometimes it is very complex, sometimes less so, but it all fits together perfectly and nothing seems out of place or shoved in.
I'd also like to mention that his guitar soloing was on a class of its own. He could swing as well as any Jazz musician, even though is solos have nothing to do with Jazz whatsoever. I mean, just, wow: https://www.youtube.com/watch?v=cqU_iZH173E
I really wish more rock musicians were like this.