While I say "spell damage," this is more often an issue you'll see in healing rather than damage dealing. What am I talking about, you ask? Down-ranking spells. Most healers who have hit the end-game understand that, with massive amounts of +healing on their gear, they can use lesser ranked healing spells to heal the necessary amount with a low mana cost -- giving great mana efficiency. (Damage dealers, alternatively, tend to stick to their max rank spells for best damage output.)
However, in the Burning Crusade, all this will be changing: spell damage and healing will be applied differently to lower level spells than max rank ones. The standard +damage and healing bonus will be multiplied by ((spell level) + 6)/(player level). (Based on the explanation given, I assume that the spell level is the level you get the next rank spell, but read into it what you will.) There's already outcry amongst the healing classes, who say that downranking is necessary to conserve mana throughout some of the game's long boss encounters -- and that this change will simply force them to use mana potions every time the 2 minute cooldown is up. I, however, am trying to reserve apocalyptic judgments until we've gotten a better feel for how this change will play with expansion encounters.