Here's my Slytherin theory.
Almost all Death-Eaters were Slytherin for the same reason why almost all Mussolini supporters were Italians. People from different houses just tend to stay together, especially when organizing a major conspiracy. If Dark Lord was a Hufflepuff, most Death Eaters would be Hufflepuffs. Dark Wizardry is no more inherent character of Slytherins than fascism is of Italians.
Seriously? But, but... Hufflepuffs would suck at being Dark Lords. There are important traits that Slytherins have that Hufflepuffs just tend not to have.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: