Historically, tier bonuses for shadow priests have been pretty boring. From tier 4 onward, we've had very little to look forward to beyond mana cost reductions, spell-specific damage increases, and extended spell durations. These bonuses were valuable, to be sure -- we could all benefit from an extra DOT tick -- but nothing was game-changing.
Towards the end of Wrath of the Lich King, though, things began to change. The tier 10 four-piece shadow priest bonus, a .5-second reduction to the Mind Flay channel, was a literal game-changer. It put Mind Blast on the back burner. Some shadow priests only cast it to refresh Replenishment; others dropped it from their rotations entirely. Whether or not the tier 10 bonus was a positive thing from a gameplay perspective is debatable, but there's no debating what a powerful force it was.
Now, after a relatively tame set of tier 11 bonuses to start off Cataclysm (damage increases -- oh boy!), game developers are dropping a pair of nuclear bombs on us. The tier 12 bonuses for shadow priests are compelling, controversial, and game-changing -- all at the same time.