I did notice that blizz cheated a little and nerfed XP when mob-tagging out in the world. In vanilla, as long as the low level hit something first, if someone else's pet killed it then it would still give the low level full XP. They nerfed this in (insert whichever boring expansion here) so I think the only ways to powerlevel an alt now are:

1) be a healer, especially a druid with thorns, and just heal them from out of group
2) run them through instances, but who knows if that's been secretly nerfed too

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