You are not starting over because you don't have to delete your original character. Many, many people find it fun to have alts and want to see the new low level quests zones as well. They may not level that character all the way up to 70 but they will play those zones and mess around with the new content. Recent changes to some of the "forgotten" classes has created a demand for people to start an alt and check them out (Pallys, Druids, warriors are next). Jewelcrafting is another reason. There is a lot more to WOW than being in a raiding guild. Almost every guildy I know has an alt of some kind on our server and on different servers.
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