Don't get me wrong, I love my D90, but after living with several frustrations for a while, I decided to upgrade.
- Focus. I had problems with the outer autofocus points of the D90. Usually, they'd work fine, but often enough I'd end up with slightly out-of-focus images. I usually didn't notice this while shooting, but only after importing to Lightroom. Grr.
- Buffer. I don't burst-shoot in the studio, but I do shoot a lot of images quickly sometimes. Especially when the models are "in the zone". It was extremely frustrating to press the shutter, and get no picture because the buffer was full. Saying, "Perfect! Hold that pose, don't move!" to a 6-yr old is pointless.
- I like new toys, and this one has:
- A FAR better viewfinder- 100% coverage and so much brighter! I can't emphasize this point enough, looking through the viewfinder was like love at first sight...
- Dual memory slots (I'll use in backup mode.)
- Loads of button/knob controls, vs. menu selections.
- A more solid build, and weather sealing.
- More, but the above are what mattered most to me when making the decision.
I did consider the full frame models, but decided against them based on price, the DX lenses I own and love, and the reach of the DX sensor for sports and macro shooting. And I don't shoot that much in low light. Maybe someday...