The
Edition War is waged all around us on the Internet and in real life, often at our FLGSs and the game table. At its core, it's all about "gaming purity and the fear of the Other."
It started (as far as I know) when 2e came out and gets sparked again and again with each iteration of D&D and spin-offs. The war is fought by those who believe one Edition is clearly superior when compared to another Edition and they believe this notion must be spread to others. Criticisms to a favored Edition are taken personally (declarations of war), and retalitory strikes are made against the opposing ideology.
Casualties in this war include threads on various messageboard escalating into grudge matches that can be summed up as: "X Edition is clearly better, while Y edition has all kinds of problems that no sane person would want to cope with"

which is often followed by "What are you talking about? Y Edition is cleary better! Only an insane person would pay X edition!"
Other casualties include: irate moderators of said messageboards, gaming groups getting broken apart because players are split on two or more editions, gaming stores losing money because a purist stands in front of products of a given edition and griping to anybody who'll listen, and lost friendships because of the occasional slap on the head.
Gaming purists are partly justified in their fears. When a new edition comes out, it represent change in the hobby. A conservative response is only natural such as "I don't want to spend money on X Edition, I like W Edition just fine--I have a lot of money invested in it." Hardly anybody wants to see their gaming dollars go to waste on a game nobody might play. This includes the gaming purists who embrace a new edition.
The Edition War boils down to people absolutely refusing to play a given edition and verbally attacking anybody playing that given edition. It
can even get to the point of refusing to play at all, rather than give in to playing said edition.
Frankly, I'd rather play D&D than not play anything at all... even 4e
But the fires of the Edition War shall continue to burn as long as Editions of D&D are released. Because there will always exist gamers who view their favored edition as an ideology...