Nostalgia has got nothing to do with it, otherwise I would be pre-ordering Legion, since Illidan is coming back etc.. wouldn't that be playing because of Nostalgia?
I like Vanilla because it is a better game than the current product, simple.
I want an MMORPG to be challenging and to have a sense of agency and a sense of achievement, oh and yes, I almost forgot, a sense of community (you know, because you're playing it online with other ppl in the same world/place) otherwise I would be playing a single-player game XD
Vanilla HAS THIS, the current product DOES NOT.
This is what I found on Nostalrius. This is why I want to play Vanilla over the current product.
If the current product was like Vanilla, then most ppl would be playing it, despite the fact that Blizzard went ape shit with the story line in Cata/MoP and WoD, people would still be playing the current product if it was like Vanilla.