This opinion presents a more challenging and thus probably less popular notion.
I think it would make for better expansion transitions if the player had to finish all of the dungeons and raids in a particular expansion before moving on to new expansion(higher level) content. This would force the player to take on the boss of the expansion at the end of it which would grant have more story resolution. This isn't to say that a player should have to beat Deathwing before creating a Pandarin. You just won't be able to access Pandaria until you beat him. I understand that a lot of people would gripe and moan, but I think it would add (force) more content into each player's experience. It wouldn't necessarily have to be so strict either. Perhaps you just have to beat the bosses on one character in order to skip the raids from that boss's expansion with other characters.