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Realize that comparing planets is hard, but is Earth really that unique?

More to the point, if everything say 20-km below sea-level up into the exosphere disappeared (and the moon too) - what would be left to differentiate Earth from other planets?

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Special is such a loaded term, unique might be preferable. –  Siv May 4 at 9:38
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I would prefer if we would stick to metric units. Also you accepted the first answer given only one hour after you asked. For a broad question like this, I would at least leave it open a few days. You are effectively silencing a whole set of different opinions a broad question like this could have attracted. –  Spießbürger May 4 at 18:29
    
@Spießbürger: (1) Switched it to metric units. (2) Post a better answer, and I'll select it, otherwise, I don't see the point in speculating that better answers might have been posted if I had waited to select an answer; the answer selected can always be changed. –  blunders May 4 at 18:38

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Most notably, a moving Lithosphere. Shaving off the surface 20km (Lithosphere is~120km) would not stop plate tectonics. What makes Earth's surface so different is that it has a constant cycling of lithosphere; Dead planets are forced to keep surface scars caused by major impacts and weathering. To this date, we have not found another example of plate tectonics, though our ability to confirm this on other planetary bodies is limited.

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What about Venus? –  blunders May 4 at 4:38
    
There is some debate whether Venus had plate tectonics in the past; I'm a skeptic, as I think venus is a bit too hot. What is not up for debate is that the planet is currently dead. –  Neo May 4 at 4:48
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Also, see "UCLA scientist discovers plate tectonics on Mars," any comments? –  blunders May 4 at 5:12
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Most notably, a moving Lithosphere ... and an active and strong magnetic field. Venus and Mars have virtually no magnetic field. Mercury does have one, but it's only about 1/100 as strong as that of the Earth. Regarding a moving lithosphere, water has long been regarded as an important lubricant for plate tectonics. Would we have plate tectonics w/out oceans? (but also see Fei et al., "Small effect of water on upper-mantle rheology based on silicon self-diffusion coefficients," Nature 498, 213–215 (2013)). –  David Hammen May 4 at 14:13
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I think its unclear about the water and the plate tectonics David. While I understand the argument, in my computer simulations I haven't needed to account for water content (or the crust) to get viscous decoupling between two plates. The most important factors seem to be the kinematics and the buoyancy of the plates. Surely the surface would eventually cool. Also, there are other planetary bodies with magnetic fields, like ganymede. Either way, the "Rare Earth" theory provides many ways that the Earth might be unique, and that it includes its interior. –  Neo May 4 at 18:47

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