9) Its pole is tilted more than Uranus's (122 vs. 98 degrees)
When you look at a classroom globe of the Earth, one of the first things you notice is that the Earth is tilted. Its axis of spin (the line through the Earth connecting the north and south poles) is tipped by about 24 degrees to the plane of its orbit. We think this may have been due to the immense grazing impact of a planetary-sized body shortly after the Earth formed. This impact blasted off debris which eventually formed the Moon, but also slammed the Earth off-kilter, giving it the current tilt.
By measuring the rotation of the planets, we've found that all of them are tipped a little bit to the plane of the solar system (except Mercury, which is in the fierce grip of the Sun, so
tidal interactions have forced its axis to be perpendicular to its orbital plane). Uranus is tipped by more than 90 degrees, so in a sense it orbits the Sun on its side.
But Pluto outtips Uranus -- its tilt is 122 degrees. Something must have whacked Pluto pretty hard to tilt it that much. What this means is that from our point of view, we sometimes see Pluto looking down on its equator, and other times down on its pole. In fact, back in the 1980s we were seeing Pluto's system of moons edge-on, as it were. Charon would pass in front of and behind the planet, an event called mutual eclipses or mutual occultations. Those were used to make maps of the surface of both objects in the same manner as described above in #8.
So Pluto is slanty, even more than Uranus. But Venus has them both beat: its orbital tilt is 177 degrees-- it's actually almost completely flipped over! In other words, it rotates backwards, east to west. Whatever happened to Venus in its distant past must have been colossal and catastrophic. Flipping a planet all the way over is
hard.