I would be careful with statements such as "4 GB is enough for everyone", Bill Gates is (in)famous for such an argument.
Well, in my opinion, 4 GB is ridiculous considering memory prices these days, and even more absurd is the fact that the Kindle doesn't have a MicroSD slot. I really doubt Amazon engineers would have trouble adding a card slot without increasing the physical size, so it's probably a market decision.
Although I am definitely going to buy a Kindle 3, I don't really like Amazon's policies. They are starting to look a lot like Apple, and that is a very bad thing.