Groups of Words' anonyomous blogger bought up a rather interesting theory regarding World of Warcraft expansions: They're actually more like sequels, in that they change a lot of the basic groundwork of the game and the classes to such a point that it feels like a completely different game.
As evidence of this, he puts forth a large list things that have changed between original WoW and Burning Crusade. Being a semi-old-timer to WoW, I certainly recognized pretty much all of the changes, and thinking back, a lot of them have been doozies.
40-man raiding to 25-man raiding, for example, really changed the face of the raiding game in a lot of ways (I actually like how he compared 40-man raiding to today's battlegrounds -- something many players do not because they enjoy it, but because you can get epic loot while being mostly AFK).
Itemization changes are a bit of a doozy as well. Those alone have changed how many classes play on a basic level. Feral Druids are still trying to assess the fallout of converting to Rogue gear in Wrath, and may need some basic mechanics changes because of it. Enhancement Shamans have found themselves in a similar, if less world-rocking boat.
Tanking is a third example that's also pretty massive. Originally, most people considered Warriors the only viable tank for any instance or raiding content. Some particularly obstinate Feral Druids and Protection Paladins might carve out a niche at the 5-man level, but it was generally agreed that it was because their guild and/or friends were humoring them.
But look at the expansions. In BC, Feral Druids and Protection Paladins are consider equal tanks to Protection Warriors for a lot of content. In Wrath, Warriors, Druids, Paladins, and Death Knights of any spec are supposed to be able to tank basic content, and tank-specced members of each class should be considered equals at raid and heroic tanking, according to Blizzard's new philosophy.
All of these changes really do seem incredibly radical, and if you've been watching the game as long as I have, you can see how it's all affected the way people play and think about the game world and their chracters -- and all this is just the tip of the iceberg. Really, both of WoW's expansions have changed or will change the way we look at the game in massive ways, so I have to say that I can see the merit of calling them full fledged sequels with backwards compatibility to their predecessors.
So do you agree with GoW's premise? Are BC and Wrath so earth-shattering as to be better classified as sequels to WoW rather than expansions? Either way, are these major changes to the game each expansion a good thing or a bad thing?