digital compass ( come on nokia why did you have to screw augmented reality for the N900?)
additional keyboard row
d-pad
hot swappable sd slot that doesn't require some parts to be removed (look at the SX1)
micro a port instead of micro-b
USB Port on the right side of the device, instead of the left
One dedicated hardware home-key
USB-otg
mini hdmi instead of component
bigger battery. Size-wise, the one from the E61 would already fit in to the hull