I've also just upgraded, though to a D90. Also using the D70 as backup. For me it's the AF-on delay that pushed me to upgrade. The main benefits with the new cam are the high-ISO performance and lots of custom settings.
The SD door is too loose, similar to the D70 CF door, but the D70's door was in a bad position. The D70's mode dial was too loose also; the D90's is better (though I don't like the shiny plate).
If my D70 wasn't slow with the AF I might not have upgraded.