Mobility, Stealth, and CC at Worlds
In looking at the champions played at World's, one thing that jumped out at me is how valued mobility was. To try to capture this with numbers, I went ahead and classified each champion as having:
(i) High mobility (e.g. having two plus mobility abilities or an ability with a reset) Lee Sin Kha'Zix Yasuo Tristana Fizz Zed Kassadin Riven
(ii) A flexible dash/leap/blink (e.g. non-targeted) Lucian Corki Nidalee Jarvan IV Ahri Caitlyn Vayne Lissandra Vi Ezreal Gragas Graves
(iii) A targeted dash/leap/blink Maokai Irelia Thresh Braum Elise Ziggs (I'm counting this as targeted because he doesn't really have enough mobility to warrant being in the next tier up) Rengar Akali Katarina Pantheon
(iv) A team movement speed buff Orianna Lulu Sona Zilean Jayce Kayle
(v) A self movement speed buff Rammus Singed Blitzcrank Janna Ryze (no one would really consider him high mobility but he's strong for different reasons anyway, and I do think the mobility on his ult helps him a lot).
I put each champion into only one category, with higher categories getting preference.
Champions in the first category were picked 190 times during Worlds, while all champions that did not fall into one of these categories were collectively only picked 165 times total. Of these 165 picks, 25 of them were Rumble, 24 of them were Twitch, 26 of them were Nami, and 12 of them were Morgana.
Further, of the champions who did not fall into one of these categories, the only champions to have a positive win rate were:
Rumble - That ult. Twitch - Has stealth and a movespeed boost in stealth. Morgana - That snare. Evelynn - Played twice; has stealth and a movespeed boost in her kit. Fiddle - Played once; that ult. Swain - Played once.
Further, of these immobile champions who didn't have a positive win rate, for the most part they weren't even close to having one. Kog'Maw came the closest with a win rate of 43%, with Talon not far behind at 42%. After this it drops all the way to Nami at 38%, and it gets truly bleak after that.
It gets even worse than that though, because the categories I've made obviously aren't all equal in terms of mobility. The champions in the movement speed buff categories mostly have negative win rates as well, with some notable exceptions. The only champions in these categories with a positive win rate are:
Orianna - Win rate of 51%, with a large part of her power coming from her ult combined with her utility. Jayce - Win rate of 58%, with acceleration gate being used WAY more for poke than mobility. Janna - Enough said. Ryze - Enough said. Rammus - Played in two games. Singed - Played in two games.
There were 78 games played at Worlds. There were 10 champions played each game, thus there were 780 champion "plays" during worlds. Of these plays, 435 of them, well over over half, were champions with high mobility, a flexible dash/leap/blink or a targeted dash/leap/blink. Thus, on average, in any given match, between five and six of the ten champions in play had at least one dash/leap/blink.
Add in Janna, Ryze, and Mao and it's not hard to figure out why melee champions weren't played, or got destroyed if they were (hi Mundo).