They hated Janna's E because it had no counterplay and benefited a passive playstyle with shielding your ADC on cooldown (ish, we all know the players who did and who didn't play like this) to keep buffs on them with maximum overtime.
They hated Janna's Ult because it was the definition of a cockblock, knock people away (R.I.P small walls) and heal them to essentially create a "safe zone" in the middle of a teamfight, or under tower to completely negate a tower dive (back when towers were more feared and tower diving was not done the way it is currently).
They hated Janna's Q because it allowed her to completely shut down an engage or cancel a high impact move relatively reliably (Malz ult, Rengar, Leona, etc.).
They hated Janna's Builds because her kit being well designed and balanced to stand the test of times and different meta's she could build pretty much anything and have it still have a use case (remember when there were players who would build frozen heart and randuins omen and go a tanky Janna?). This allowed Janna (and still does) to opt in to building the most gold efficient or situational items to remain relevant (in my opinion Janna is still the best character to buy Mikael's on since it's lack of stats aren't missed as much on a Janna as they would on another character who is item dependent).
They hated Janna's playstyle since she rewarded the playstyle of an eraser. Janna's job was to cover up the mistakes of her team while working to clean up the edges of a well executed teamfight.
Now, they hate Janna's W the one skill that passed through the times remaining the poster child of "passive power" since the removal of Zilean's % XP increase and Sona's Auras. It now is targeted damage that causes her to lose movement speed and open herself up to more pick potential. Sona hates us because our W does more damage then their Q passive, Lulu hates us because we don't have to sacrifice our defensive capabilities to poke effectively, Karma hates us because we don't have to hit skillshots to chunk players out of lane early on.
To tell you the truth I don't have an answer to keep Janna how we love her without stripping her of her current kit. The only idea I have is to have the W upon cast slow her by 8% instead of increasing her movespeed by 8% to open her up more to an aggressive playstyle while keeping the damage where it is. My main point with the mini rant is that because Janna's kit is not item dependent and well crafted for her purpose players will always hate her because she has the ability to suit different playstyles and situations and in theory should also be a solid pick.