This is really hard to say because overall, I LOVE my Kindle. I don't expect this device to be all things, it does exactly what it was sold to do, a device to read on. Even with the Nook out there and other e-readers on the horizon, I still love my Kindle. Having said that, there are two things that I don't like and here they are (I am sure this is no surprise to anyone):
1. Lack of content management. If we got folders so I could organize the over 500 books on my Kindle, beyond the title, author or most recent sort, I would be in heaven. This appeals to my OCD nature. Sorry even my DTB books were organized like they are in a bookstore or the library. It just makes things easier to find.
2. Expandable SD slot. This was a feature on K1, not a big deal, but would be a nice to have.
#1 is the primary thing I hate about my Kindle. In the grand scheme of things, it is small, but just something that would (IMHO) move the Kindle from good to great!