http://www.minecraftwiki.net/wiki/Stairs
There is a glitch where when stairs are placed at the water's edge,(with the top edge of the block at the water's level) Any boat going at any speed can collide with it with no damage to the boat, making it ideal for the creation of a dock.
At some viewing angles, hovering the cursor over the stair block would make the black outline flicker, making mining the stair block glitchy, and the block behind it would often be mined instead.
Stairs used to face a direction automatically when placed. They would attempt to face a block adjacent to the stairs, so as to create a climbable surface towards that block. However, this was often more annoying than helpful, and was therefore changed to be placed facing the player.
From Alpha version 1.2.0_02 to Beta 1.5, stairs had the odd property of being transparent to light; when placed one or more spaces above the ground with air beneath, sunlight or other light sources would shine through and light the blocks beneath/behind the stairs exactly as if nothing opaque were present between them.[1] Other atmospheric effects such as snowfall did not penetrate stair blocks, making them an unusually effective material for creating roofing which kept weather out, but still let daylight in. Snow could not accumulate on top of stair blocks, grass was able to grow on the block directly underneath a stair block, and grass wouldn't disappear if a stair block was placed on top of it. In Beta 1.6, the icon for stairs was reversed, however nothing else to the properties of stairs was changed.