There have been a few posts about this recently.

http://www.reddit.com/r/AskHistorians/comments/1stom4/how_did_austria_manage_to_not_be_split_like/

TD;LR Austria was split from 1945 to 1955 and was reunited in 1955 under the condition they remain neutral. I couldn't find the specific thread with this reply, but I remember someone posting that Russia used the reunification of Austria to gain advantage in negotiations with the Allies. Austria is also not as strategically and economically important as Germany.

Unlike the territorry that became the DDR, the Russian zone of occupation of Austria was not economically or strategically useful to the USSR to the same degree. In terms of geopolitics, East Germany (the DDR) was right on the northern european plain, the highway of invasion between Eastern and Western Europe since before the days of Napoleon. In contrast, Austria is not very dangerous as an invasion route if the defender controls the mountain passes in the Carpathians and the "Danube gap" which the Soviets had effective control over via Hungary and Czechoslovakia.

From an economic standpoint, an independent Austria, as long as it was not part of Germany, was too small to pose a military threat to the Soviets. Without Moravia and the Pannonian plain as an economic hinterland, Vienna is just a nice mid-sized city, rather than the capital of an empire.