Game-wide mortality.
The common reason I have heard against this is that players are heavily invested in their characters. As someone who is very heavily invested in my characters, I would agree with that. However, I believe that this heavy investment is ultimately a bad thing for the game. When players are so unwilling for anything bad to happen to their characters, they end up creating a very stagnant atmosphere. Players will go to extremes to preserve the power or influence of their characters if that character represents years and years of investment. However, it is my assertion that if a player understands that their character could die at any moment, they will do more to "live in the moment" and make things interesting instead of creating long-standing dynastic hegemonies that prevent others from enjoying the game.
Will this cause some players to leave? Absolutely yes. But, I believe that in the long-term it will create a much better atmosphere for the game in general because those players that remain and the new players that join following this change will be able to play a more fluid game with deeper interactions not limited by long-standing friendships, alliances, and entrenched leaders.
