As written by many others as well, my main complaint is the dynamic range. I tested D800 for few days, and when using ISO 100, I definately could see the difference in dynamic range in favour of D800 over 5DmII (some say 2 stops, I say it's probably about right). I prefer to use low ISO anyway and landscaping so Nikon would probably be the smarter choise for me. It's just too expensive to switch camps just like this. Canon EOS 3D will probably remedy this "issue", but it's still a long way for release of next generation high megapixel Canon DSLR (next year?).
And in conclusion: the difference between Canon and Nikon bodies are overall, very marginal. Every body has disadvantages and advantages, it's the consumer's choise to choose the preferred gear. I'm just saying there always should be room for improvements, so Canon does have the advantage of beeing the market leader so they can hold back in technical innovation to observe what competitors are doing while still holding the market leader status.