tbh ive done more exploring of the changes on my main with a few guildies we were runnin round the new zones exploring the changes.
I fully get what your saying about making the levelling slower if you wanted it that way, i mean they added an option to turn off xp in bg's so if they had an option to reduce levelling xp i can see that being another option for people.
Personally when i level i dont immediately leave an area once ive passed the level bracket. If im enjoying teh area ill stay till ive cleared pretty much any quests i can find, i can certainly see myself doing that in the goblin area and ashenvale.
Once we hit outland all i want to do is gtfo there and the same with northrend, those areas are the grind now.
I think a lot of people will have my mentality of exploring the old worlds on their main to see the changes, any of us with 310% riding skill will be able to explore areas till their hearts content and at whatever speed they choose.
My reason for rerolling is to be a goblin so i cant do that until after my main is 85 and raid ready which will be some time.
By the time im ready to start on the goblin ill be sharing my time between the new alt, leveling up key professions on other alts, making gold for my main, raiding on my main, helping guildies level etc.
Its a lot of a time sink for sure and what little time i will have on my goblin i will enjoy very much but it will def be with hierlooms, as much for their gear as their xp.
Id say overall across my main and alts there wont be an inch of the new content that i dont experience in some shape or form, it will just be across a range of chars rather than one alt.
Hope that makes a bit more sense, its not as much about flying through content per say as seeing what you can, when you can.