Tbh, it's only fair that Cataclysm favours Horde.
Vanilla was Alliance favoured, you had lots more outposts around the zones and even in Horde zones you had quests, while Horde didn't have much outposts.
BC wasn't really one-sided, but it was very slightly Horde favoured; Alliance and Horde both had awesome quests, but Horde were more involved, having a few cutscenes, and also the world being flooded with blood elves and Nagrand with orcs. Still, that is what you expect from the orc homeworld, and you should also expect blood elves since they make up half of illidan's entire army.
WRATH was the same as BC, except more alliance favoured. alliance got slightly more epic questlines, and the whole expansion is based on a human prince who got corrupted.
Because of this, it seems right that CATA should be slightly Horde favoured. Blizzard made it Horde favoured; Silverpine, Azshara, Stonetalon and Andorhal questlines all rock. Still, that doesn't mean Alliance don't get any gain; Alliance get part of Southern Barrens, and the questlines have still been revamped to make it better.