How about: "I really love your books. They're great. There's just... well... one little thing... You really, really, really need to work on your character development. Sure, the science in your books is intriguing and frequently I'm in awe of the amount of research you put into each novel - but DAMN IT, man, your characters are so one-dimensional you could plot them on a single axis. Seriously - when I read 'The Terminal Man' I really, honestly tried to feel some sort of sympathy for the lead female, but it never came. Why? Because even despite your weak attempts at fleshing out her private life it was so incredibly transparent that I had to put the book down and get a glass of water. Or what about 'Sphere' - what the FUCK was going on there? You had these people stuck together in a small space thousands of feet underwater and you barely even began to explore the cabin-fever angle. Yeah, they turned against each other and stuff but THERE WAS SO MUCH UNTAPPED POTENTIAL IN THAT BOOK IT MAKES ME SICK! And don't get me started on 'Jurassic Park', that just sucked. And 'The Lost World' - WHAT THE HELL WERE YOU THINKING? Talk about selling out! Don't you have any sort of integrity AT ALL? Isn't it enough for you to be a crappy writer that somehow manages to make a packet out of selling your unbelieveably scientifically accurate but unbearably sterile SHIT? YOU MAKE ME SICK!"

*ahem* Or something like that, anyway.