I think this expansion, I'll likely hold off on upgrading my wow's immediately.
I'll likely still pre-order and pick them up on day 1, to have them ready.

But then run dailies for 2-3 weeks, on every team available.
And finish off the leveling for any team I enjoy that is not yet 80.

Then, with significantly fewer people in the zones, give it a shot.

Never know, the temptation to go fast might kick in by then.
But that's not the plan, at this point anyway.