The only things I can think that I might not like... (and they're just little things)

I probably don't need such a big phone.

I don't care about 4G LTE so the Lumia 800 might have better battery life even though it has a smaller battery.

I like that the Lumia 900 has a larger screen but somehow it doesn't seem as impressive when it's not slightly curved like on the 800, but who knows, maybe it's a better screen than to 800 but nobody knows yet because they haven't had a chance to actually test one.