Minecraft wiki entry for Slime_Block, associated text: A minecart leaving a rail onto a slime block can continue on a rail on the opposite side if it has sufficient momentum leaving the initial rail.
Slime blocks under soul sand make the player move slower than if the player was just walking on soul sand. This is also apparent with ice.
Entities launched upward by a slime block on top of a piston achieve slightly more than 6.4 blocks of lift, including the lift from the piston. Entities launched sideways across normal blocks or air are moved 3.510 blocks in the direction of the launch.
While falling, placing a slime block while just before impact prevents all fall damage.
Slime blocks are invisible if seen from the clouds.
Players walking on slime blocks move at 1.359 m/s, a speed reduction of around 70% from the normal walking speed.