13%

Some recent cataclysm previews have shown that the current 13% spell damage buff is going to be dropped to 8%. While this is a step in the right direction, they need to take another step before they are done.

As I pointed out, the 13% spell power buff gives approximately 8.5% more damage to a typical raid of mixed dpsers while the next best buff is around 3.2% extra damage to the same raid. But decreasing 13% to 8%, other things being equal, it will now give a 5.2% damage bonus to a typical raid, so about 63% more than the next best buff.

If they dropped it to 4% instead then it would be a 2.6% buff to raid damage, making it well above average in terms of raid buffs. At 5% is would still be the most powerful raid buff, but by a very small factor instead of by a very large one. I hope that cataclysm reduces this buff to 4% or 5% and gets it under control so that a raid without this buff can function, much as a raid without any other particular buff can.