I actually think that people's "first novels" tend to actually be better than much of the rest of their work, for the same reason that a band's second album is often inferior compared to the first: you've had all your life up to the point the publisher accepts it to polish and work on your first novel, whereas once you get into a publishing contract deadlines can start to rear their heads, you've got people writing in begging you to bring your second novel out and/or second-guessing every decision you made in your first novel...

Plus, if the author has a few good ideas, in the first novel they're fresh and original and by the second or third the "ooh! shiny!" factor wears off...

I think it's even worse in the fantasy/SF genre if you're writing multiple books in the same setting . You get to a point where everything is a little too detailed - little mysterious things that sounded really cool when the writer obliquely referred to them in book 1 are raked over and explored exhaustively, to the point where you're sick of hearing about them.

Yep - alternatively you get so damn famous that nobody dares edit you anymore. And we all know how well *that* works. But to be fair, I think it does depend on the author. Sci-fi/fantasy seems to bring out the worst in people but actually, in the normal course, you might think writers would mature like fine red wine over the course of several books.