I was playing Super Mario Bros. 2, and going through the desert levels, when I realized how awesome quicksand would be in Minecraft.
Slightly darker colored sand blocks that fall, just like regular sand blocks. Standing on one of these sand blocks will cause you to sink slowly down, threatening suffocation. You can escape a quicksand block by jumping repeatedly, which will keep you above the surface enough to make your way back to the edge.
Basically, you make a moat of quicksand, with a wall of cacti next to it, protecting your house. Anything that comes towards your house steps in quicksand and starts falling through it. If they keep trying to move forward, they get hit by Cacti. If they stay in place, they sink, and get burnt by Lava. Their only choice, for survival, is staying away.