Not sure that they slowed down the leveling or not, but from the looks of things, I kind of doubt it. THe server I usually play on, Spirestone, had its first level 80 earlier today, followed by several more. Apparently the new achievement broadcasts to all areas when realms firsts (such as the first level 80, the first rogue, the first human, etc) occur. Unfortunately, I'm at least 2 days away from starting in Northrend.

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To those that understand, I extend my hand. To the doubtful I demand, take me as I am!