Personally I voted for 1-3x, my thoughts are as follows.
Vanilla was by far my most memorable days in wow, but when at lv60, I was so burnt out of leveling by around 40 that I felt it became more of a job than a fun thing at that point. Leveling vanilla becomes extremely repetitive due to the nature of how basic the quests are.
The 3 main type of quests you do are so repetitive, it makes leveling feel like a job. After about 30 levels you definitely feel burned out from the same thing over and over.
- Kill X mobs
- Gather X
- Deliver item to X person
Then you get the rare unique quests once in a while.
I just feel people will be so happy at first with it bringing back all the old memories, but it wont last too long until you start thinking 'ok im so fed up with leveling' I just want to be 60 now.. and cant be bothered to continue, and quit half way through the grind.
Now thats only speaking for a portion of players. Die hard vanilla fans will stick it out, as will I, whatever the XP rate is at.
People have just leveled so many characters in the past, that the average player is excited about being level 60 to do the old raids again, but will find it hard to put the work into getting there again.
I think doing at least 2x or 3x will give those people who weren't willing to play 1x enough reason to now play. And those who wanted 1x I cant see it being a deal breaker and losing those players, as much as they may say they wont. The high level of scripting is what will matter most to everyone.