Everything you knew about raiding is history

I guess it's saying something when the biggest negative change an expansion brings is that it indirectly makes a lot of old endgame content obsolete almost overnight. Before Burning Crusade, transition from level 59 to 60 was the point where a completely new game started. At 59, you were in the little leagues, doing quests and five-man instanced dungeons. At 60, you were expected to move to raiding; playing with more than one group of five players to tackle the hardest stuff the game has to offer. Problem was, the gap between running, for example, Scholomance (easy five-man instanced dungeon for level 58-60) to clearing up Zul'Gurub (a 20-man level 60 raid instance) was so huge, that many players could never manage the transition.

While seasoned players might say that an instance like Zul'Gurub was "easy", it was essentially first area where you were expected to play as a part of a larger group of players, use voice communication tools such as Ventrilo or TeamSpeak to coordinate play, and spend time learning complex boss encounters with little margin for error.

The Dark Portal to the Outland on the launch day - bit crowded, but the servers worked surprisingly well.

With Burning Crusade, this transition at 60 is gone. The first quests of Outland, reachable at level 58 if you so choose, immediately start providing weapons and armor that match the things that used to be obtainable only from 20- and 40-man raids. This means that a large portion of World of Warcraft became obsolete overnight. There is simply no reason to enter any of the old raid instances any more - Molten Core, Blackwing Lair, Zul'Gurub, Ruins of Ahn'Qiraj, Temple of Anh'Qiraj and Naxxramas all have loot that is no better than things you can get from Outland alone or with a small group.

It's true that if your character was completely decked out in the best loot from Naxxramas , you would replace most of it only once you hit level 70, but that's only because most of the high level items that are easily obtainable in the Outland are designed to be just about Naxxramas epic level quality - not good enough to swap in for the best epics due to the costs of obtaining new enchants and to cover the loss of set bonuses, but otherwise on par. Those with characters wearing things like the Tier 1 and Tier 2 epic sets from the old raid dungeons will end up swapping most of their items for better ones from quests at around level 68-70.