In the past few months, or I support over the past year is more accurate, I've been working towards loremaster achievements with alts. Particularly with the new allied races and the "new" level scaling changes.
I've run into a couple of annoyances: group quests for old content than cannot be soloed, and outleveling zone before finishing the content. Both can be worked around, but both also sort of defeat the purpose of level scaling. I can wait and come back when I outlevel the content in the case of group quests, and I can just keep completing gray quests and killing green mobs when I want to stick it out and finish a zone.
Ideally the group quests would be nerfed and/or made to be single player, and the level scaling scale to a higher level, just another 1-2 levels would be fine.
If they really wanted to get fancy they should dynamically change the level scaling cap based on what content you've played. You could level up to the point of the current content (the character boost level) in whatever old content you wanted, but killing mobs or completing quests in the next content tier would snap the scaling for the previous content tier down. For example you could quest to level 83 in vanilla content, but as soon as you started questing in Outland or Northrend the level camp for vanilla content would snap back to 60.
The whole point of the level scaling is to be able to quest along happily in whatever content you are working on without feeling you need to leave for greener pastures to actually progress, and right now that is broken along the boundaries between content tiers.
Some sort of compromise could be an NPC in Stormwind to toggle level scaling caps on or off. That seems to be the go to solution for similar things.