Wow, you not had your D90 that long have you? :nuts:
I've probably noticed more change than most people upgrading to 5Dmk2, as I've come from a 350D, rather than coming from a 40D or 50D. Still earlier days, so far these are some of the things that have really stood out and obviously some of these are nothing to do with FF :

Image sharpness, most images need little or no sharpness and the detail can be clearly seen even on smaller resized images (see the lines on my abstract art image this week).

Potential to crop by huge amounts and still have a nice large image is amazing!!!

Large bright view finder is a real bonus, compared to the tiny pinhole view finder on the 350D.

VGA screen (your D90 has 920k LCD screen the same), is a revelation compared to the tiny 350D LCD.

ISO range, means I can take shots that are keepers in lighting conditions that were just not possible before.

Auto ISO setting, think most newer Canon's have this now (and I guess Nikon's too), but it really helps when you grab the camera and don't have time to check the settings. Plus having an Lcd display on the top with all the settings is great, the number of times I forgot to change the ISO setting on my 350D when from a previous high setting was many, sounds silly but have wasted some decent shots because of this in the past.

Lack of post processing required. I found with the images on my 350D I was really starting to spend a long time in post processing in PS, amazed at how little I need to do now, feels really odd.

Build quality and feel of both the camera and lens.
I guess Chris can be more specific for you, owning a D700.