The biggest improvement over the 1D II is, in my opinion, the 2.5" LCD.
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The Canon EOS 1D Mark II N features a new "Magnified View From Any AF Point" option that sounds very useful for checking sharpness. Instead of zooming in (during review) and scrolling to the AF point used, you can now zoom in at the AF point used. Magnified view is now available during quick review as well.
The frame buffer can never be too large. I'll take any improvement offered to me in this area - especially when the frames go by at 8.5 per second. However, going from 20 to 22 frames will probably not make much difference to most photographers.
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The Canon EOS 1D Mark II N now allows "split recording". The camera can save the RAW file to one memory card (typically the CompactFlash card as CF generally has higher capacity per $ and overall at this time) and the JPEG to the other (typically the SD card).