In Siege of Orgrimmar, we had a week of normals only before advanced guilds could sink their teeth into the heroic version. Considering that these days many folks look toward heroic raids as the
measure of raiding progression, it's easy to find yourself asking the question -- why would heroic modes open after normals? If heroics are really what people are waiting for, why make them wait? And wouldn't hardcore players love the chance to jump straight into a heroic with their previous tier's gear, just for the extra challenge? (Though let's be fair, how much normal-mode gear are they going to get in a single week of raiding?) Anyway, as it turns out, the answer
is quite simple:
...which makes a lot of sense. Heroic modes are more complex than normal modes by far. Not only do the bosses generally hit harder, but it's common for heroics to include new mechanics entirely. Heroic Ragnaros's legs in the Firelands come to mind, for example. Considering that raids are already complicated bits of gameplay, it's reasonable to want to squeeze every last bit of testing out of a raid before implementing the most difficult parts of it.