It is distracting and inefficient. For someone who likes to play each character to their potential (relatively), seeing a lowbie toon with the 1000 Kills achievement but discovering that they only have 233 kills pokes holes in the whole curtain. Achievements don't matter anymore, is what this says. Milestones for individual characters have been removed. So, why bother playing lowbies anymore?
When you work hard and buy a CD, you listen to it often. When you burn one from one of your friends for free, you never listen to it. You are lazy because you got it without working for it.
Account-wide achievements make players lazy. Lazy players lose interest. Panda bears and kung fu will not always be shiny. But grinding out achievements will always make people come back and play hard.