The deal with Venus: dat atmosphere. She's a true sister to Earth in mass, gravity, and composition. She's also good evidence that Earth had a vastly thicker atmosphere at one point. Right now Venus has 90x more atmosphere than we do. And look at the proportions:

    Major:       96.5% Carbon Dioxide (CO2), 3.5% Nitrogen (N2) 
    Minor (ppm): Sulfur Dioxide (SO2) - 150; Argon (Ar) - 70; Water (H2O) - 20;
                 Carbon Monoxide (CO) - 17; Helium (He) - 12; Neon (Ne) - 7

That's interesting because if you take away the products of volcanism (a major source of sulfur dioxide and carbon dioxide here on Earth), you're left with... Nitrogen... Argon... Water... and noble gasses. Look familiar?

The best thinking right now is that Earth outgassed and held on to a similar volume of gasses as Venus. But since we had a substantial volume of liquid water on the surface, the oceans naturally absorbed CO2 and then deposited it on the sea bottom as carbonate minerals. (Life eventually learned that trick too, leading to things like shells)

So limestone, dolostone, chalks, calcite, marble, etc.... all those carbonates are likely the product of our liquid water oceans drawing out the vast bulk of the C02 in our atmosphere, leaving relict gasses in its place. Same starting point but we ended up with vastly lower pressure and vastly different final composition.

Venus had no such luck. She suffocates forever.

http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.htm...