I read a cool article in the June issue of Scientific American on the "Tunguska Mystery" (this is currently only a preview of the full article, but this is also a good source). The Tunguska Event was a massive explosion that flattened trees in a remote area of Siberia in June 1908. It has been assumed that a massive meteorite or possibly a comet was the cause of this event, and the "mystery" is that no material from the object itself has been found. The few eyewitness reports described a huge fire in the sky, and subsequent research and simulations have suggested an air burst perhaps 5-10 kilometers above the surface that completely consumed the object while causing high enough dynamic pressures and temperatures on the surface to flatten and burn trees, but no impact crater or surviving pieces of the projectile.
The SciAm article is by a member of an Italian team that visited the site and identified a small nearby lake as a possible impact crater caused by a piece of the original object that remained intact to the surface. They did various tests and imagery which seem to show the buried remains of trees, with signs of a dense, solid, meter-size object that could be a fragment of the Tunguska body. They are returning to the site this year to drill into the sediment at the bottom of the lake to try to reach this dense object. If they recover a piece of the original object, it might finally provide an answer to the Tunguska mystery.
In addition to its active geology that tends to erode or destroy signs of impact craters over time, the Earth of course also has a dense atmosphere which causes smaller space debris to burn up before reaching the surface. As discussed here, a stony "space rock" (as opposed to a metallic one) would have to be about 220 meters in diameter to reach the surface - pretty big! But space objects of intermediate size and energy need not reach the surface to cause major destruction. Check out this Sandia Lab page for some cool simulation results (including videos) from December 2007. The momentum (not just the mass) of the object is clearly important!