• The D700 is a smaller, and hence lighter, body.
• There's a built-in iTTL flash on the D700 that supports Nikon's CLS in Commander mode. The D3 has no built-in flash.
• The D3 had a 5:4 aspect ratio setting, in addition to it's FX and DX modes. This has been removed on the D700.
• A single card slot on the D700; the D3 has two and can use the second for backup, overflow, or can write JPEGs to one and RAWs to the other.
• The D700's viewfinder covers 95% of the frame; the D3 viewfinder has 100% coverage.
• The D3 has a slightly more "advanced" viewfinder display; focus points aren't marked by large black rectangles on the D3 (they are on the D700), and the D3 in DX mode "masks out" the unused area; the D700 just shows a rectangle.
• Burst rate; 5.0 fps body only for the D700 (8.0 fps with MB-D10 grip). The D3 can shoot at 9fps, and up to 11fps with certain restrictions.
• The D700 has more configurable button customization than the D3 - even if you take into account the recent D3 firmware upgrade, the D700 is better in this respect.
• The D3's bullet-proof shutter is rated at 300,000 actuations - the D700's is "only" 150,000.
• "Sensor-shake" dust reduction is present on the D700, not on the D3.
• The D3 has a microphone and speaker for note taking; the D700 does not.
• There's no room for the D3's rear information panel on the D700, so much of this has been moved to the top LCD.
• The D3 supports interchangeable focusing screens; the D700 does not.

This looks like a long list, but read it carefully. Do you care about interchangeable focusing screens? Is 8.0fps (with grip) not enough for you? Worried about the "95%" viewfinder? Really, the D700 spec is so close to that of the D3 that unless you specifically need dual card slots or 300,000 shutter actuations then the D700 makes buying a D3 a much less attractive option. And then the D3 doesn't have the built-in flash, for fill or as a commander, or a sensor-shake cleaning mechanism.

• The D700 has a "full 35mm frame" FX sensor, the D300 is DX.
• The D300's focus points cover more of the frame than they do on the D700, unless you're shooting in crop (DX) mode.
• The D700 has better button customization options
• The D700 is 12.1mp - the D300 has a very slightly higher resolution of 12.3mp.
• The D700 has a 95% viewfinder, the D300 is actually 100%.
• The D700 is *very* slightly larger (see images above), and heavier - it's 1074g (37.9 oz) to the D300's 925g (32.6 oz).

This looks like a very short list. It is. What you are paying for with the D700 is that incredible sensor, plain and simple.