You were probably sure that there's some great symbolism in the fact that the Mezuzah is slanted.  If that's the case, I am afraid that I'm about to let you down. You see the tilted Mezuzah that you will find on so many Jewish homes is actually just a product of a technical debate in Jewish law.

The debate took place between the great 11th-12th century Rashi and his Grandson Rabeinu Tam.  Rashi said that the Mezuzah should be affixed to the door in a straight vertical fashion. His reasoning was that it should be placed on the door similar to how the Torah is placed in the Holy Ark in the Synagogue. His grandson Rabbeinu Tam disagreed with his grandfather's ruling, he was of the opinion that the Mezuzah should be placed on the door horizontally. He compared this to the way that the Torah is read on the Bima in the Synagogue.

Till this day most Sephardic Jews place their Mezuzah straight up and down like the ruling of Rashi. They are consistent in their reasoning because they also read their Torah scroll standing upright in a decorative case. But the later Ashkenazi Rabbis ruled that the best thing to do was to put the Mezuzah up with the top slanted in. This way you fulfill both opinions.

Now if you are on the ball you will ask, "Why Isn't slanting the Mezuzah going against both opinions, thereby not satisfying either one?" Excellent question! The reason is that Rashi actually was saying that it is Kosher as long as it is not horizontal and Rabeinu Tam held it was acceptable as long as it was not entirely vertical. Therefore slanting it satisfies both grandfather and grandson.

Of course if you are Sephardic you will continue to affix your Mezuzah straight and just chalk it up to those crazy Ashkenazim.