This problem existed in every expansion, really. Why does your DK have to go to Outland? If your main killed C'Thun way back in vanilla WoW, then why will those druids in Silithus ask my alt for help when I level him through that zone in a couple weeks?

That being said, though, if you go to the new content as soon as you can (Outland at 58, Northrend at 68), any individual character shouldn't encounter too many conflicts. Quagmirran is a complete nobody in the lore, so maybe he's been killed and a new giant moved into his bog. Exarch Maladaar is a ghost, so maybe he was beaten by adventurers when BC was new and now he's back from the dead... again. The really storyline-dependent stuff in each expansion so far has been stuff that you can and probably will skip as you advance, like the Netherstorm quest chain where your character is surprised to learn that the blood elves are in league with the Burning Legion. Well, for characters created after 2.4 and WotLK, Kael'Thas and elves loyal to him have always been evil, but that quest chain is long and has parts that require a group so they'll probably skip it anyway, so where's the conflict?

For superheroes, crimefighting is a neverending battle, there's always a new villain getting paroled from prison or a new crime boss consolidating power; for RPG players, dungeon crawls are.