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Closeness to Sun
Venus is a little more than 20 million miles closer to the sun than the Earth. This means the planet receives more intense and concentrated solar energy. By itself, this distance from the sun is not enough to create an uninhabitable environment. It does mean that the mechanisms for dissipating that energy have to work much harder. In Venus' case, there aren't any of those mechanisms in place, so all of the sun's energy is converted directly into heat.
Carbon Dioxide
The most well-studied greenhouse gas is carbon dioxide, and Venus' atmosphere is made up almost entirely of this compound. Carbon dioxide traps and holds solar energy. At the surface of Venus, temperatures hover at 880 degrees Fahrenheit. That's hot enough to turn lead into liquid. With conditions like that, obviously no life can exist. Scientists call the atmosphere of Venus a "runaway greenhouse," because there's no chance that a natural mechanism will ever reverse the conditions on the surface.
Cloud Cover and Air Pressure
The thick clouds of Venus actually reflect a lot of solar energy, but what does make it through will never get out. The clouds are many miles thick and composed of sulfuric acid. In the upper levels of the atmosphere, winds whip at greater than hurricane force. The air becomes stagnant closer to the ground. At the surface, the atmospheric pressure is 90 times that on Earth. The clouds make for near darkness all the time, and the density of the air increases heat retention even more.
Lack of Water
On Earth, water and plants cycle carbon dioxide through a system. Water traps carbon dioxide, some becoming bound up in rocks. Plants require carbon dioxide to perform photosynthesis, releasing oxygen as a byproduct. None of these processes are happening on Venus. Therefore, carbon dioxide has nowhere to go and no oxygen is being produced. Scientists believe that Venus may once have had large oceans just as Earth has, but that these boiled off as the planet got hotter.