As the title suggests I've been playing since Vanilla and honestly for me, this is the first xpac where PVP seems fun. Excluding gearing issues which is being fixed in 7.2, I have not felt invested in doing the PVP objectives in zones, or doing battlegrounds except for PVE caches until Legion hit and it's prestige system. So while I'm just getting started on the PVP road, I'm hyped for the changes coming with PVP brawls and Arathi Basin.
Key things that prevent or prevented PVP from being fun in the past is how uninteractive it used to be. I'm sure this post will draw ire from PVP players, but getting CC locked to death is not fun gameplay. Being a healer and being absolutely dead the moment you're sighted is not fun. And while ideally I don't think it's healthy to have addons that put giant crosshairs over healers in BGs.
Say what you will about this being a l2play issue, but Legion is the first xpac where I've not gotten absolutely disgusted after a few BGs of being able to do nothing due to the constant CC and being instantly blown up the moment you go near a flag.
Also rewards, they've replaced impermanent rewards with permanent rewards such as weapon skins, loads more mounts, pets, toys, etc. Also WQs populate the world with PVP objectives that are meaningful to complete while also being a way to bring PVE players more into trying out PVP than Ashran or any other PVP objective has tried to do in the past.
I'm sure this post will be very antagonizing for most folks, but I think Legion is bringing more non-PVPers into the PVP scene than ever before.