For most of WoW, the pathway is provided by level. At level 35-40, you do Scarlet Monastery, 40-45 Uldaman, etc. But at endgame, Blizzard resorts to attunements to provide the same pathway.
There are actually several types of attunements in WoW. First are the keys, where only one person in the group needs the key in order to :
- Upper Blackrock Spire
- Shattered Halls
- Shadow Labyrinth
Second are the initial raid attunments, where you need complete a non-raid instance to attune to a raid instance.
- Molten Core
- Blackwing Lair
Third are the heroic instances, where you need to run a fair amount of non-heroic instances before being allowed to run heroics.
Finally, there are attunements which require you to complete raid instances.
- Serpentshire Cavern (Removed)
- Tempest Keep (Removed)
- Mount Hyjal
- Black Temple
But how many of these attunements are really necessary? The thing is that players are pretty good at figuring out what the pathways are. AQ40 had no attunement, but everyone knew you had to beat Blackwing Lair before moving on.
Would anything really change if the keys for the first set of instances didn't exist? I don't think anything would really be different, save that it would have been a lot easier to make groups for UBRS back in the day.
To be honest, out of all the attunements listed above, the only one I think is actually necessary is the Karazhan attunement. Insisting that you have successfully done several of the regular instances before moving on to Karazhan is probably a good idea, especially for guilds and players who have never raided before. Attuning for BWL is unnecessary, as most people know that MC comes first, and Razorgore will ensure that you understand this.
As well, I think the implementation of the fourth class of attunements is not done well. It's not enough to be able to attune your current raid, you need to be able to attune your entire raiding force. For example, let's say your guild successfully defeats Kael'thalas. The next day, all excited, you head for Mount Hyjal. What are the odds that you will be able to field a full raid? For most guilds, the odds are low, because you need the exact same 25 people as the previous night. But you won't get the exact same people. A couple will not be able to log on for real life reasons, and you'll need to swap other people in. Only because of the attunement, you can't. And that's just messy and saps your momentum.
In general, artificial barriers to content are not really necessary. There are already enough real barriers in terms of level, difficulty, and required gear. Keys and attunements are not going to significantly slow down the hardcore, and they just make life more annoying for the more casual players. I think that WoW would not be hurt by removing most of the attunements and keys in the game.