[VISUAL CLARITY] Soulstealer Vayne vs Kalista
My main (and really only) annoyance with the new vayne skin is that it's going to make it really hard to tell the difference between soulstealer vayne and kalista in a clustered teamfight, especially to people who aren't familiar with all almost-130 champions or otherwise have lowered graphics. And it's not totally off-meta to have both kalista (bot) and vayne (top) in the same game.
Vayne: https://1.bp.blogspot.com/-xS-w5d7Iq-E/V2CamTJ7guI/AAAAAAAAQJo/0Mz3st9hBiwHuBNdq6oO0FkkulDtDHLqQCLcB/s640/vayne.jpg Kalista: http://67.media.tumblr.com/072eb42b549cbf0bd1523b5944ea9239/tumblr_nek3pki7VG1sajfnro2_500.jpg
The reason is that in clustered team fights, you really only see the top of people's heads. This new vayne skin, when she's facing south/down/the camera/towards the player, has a really (really) tall ponytail, similar to warring kingdoms nidalee and kalista's baseskin. This high ponytail is not shared in the other vayne skins and so isn't an immediate visual indicator that the glowing champion with sharp spiny things is vayne. In fact, the high ponytail (-esque thing) with glowy stabby things is kalista, so creating more of a visual difference between the two would be super helpful.
tl;dr: Vayne, Soulstealer Vayne, please let down your hair!