Not what I'm listening to at the moment, but a random thought that just came up. Tori Amos' version of Slayer's Reign In Blood must be the one cover that's over twice is long as the original song without actually doing anything different. I was shellshocked when I first heard it! It's haunting, it's hypnotising and it's slooooooooooooooow. Can't say I find it very good, but it's certainly something else.

Usually when you talk about Slayer covers they're actually faster than the original, which in most cases is quite an accomplishment. One that comes to mind is Luciferion's version of Chemical Warfare.