I think the negative reaction against Quinn's design is reasonable, if a bit misdirected and incoherent. We should take it seriously. This isn't about "OMG NO BOOBS". It's more thoughtful than that.
Some parts of the reaction are silly, yes. Quinn wasn't "covered up", Quinn is still a ranger and AD carry rather than a tank. But the ideas behind the criticism of Quinn's look are basically sound, though IronStylus adequately responds to them.
Two reasonable criticisms here.
First, critics of Quinn's appearance are concerned that her armor doesn't match the "ranger" trope -- and defenders of Quinn (I am one) have to grant that it really doesn't match it. The critics expected a lightly armored, leathered, more rugged tracker or agile falcon-user with green accents. Instead, Quinn has bulky and heavily padded legs, white plate accents on her hips and shoulders, a heavy gauntlet on her right hand, and a very functional but heavier crossbow. This bothers people who expected a more typical ranger in League rather than what looks like to them like a foot-soldier
We were prepped for a ranger by IronStylus. I also think that part of the argument is that these two things (typical fantasy ranger, IronStylus) run against each other -- that a character as armored as Quinn is would not behave like the ranger/tracker that she is supposed to be. Most importantly, nothing in the journal's visual elements clued us in that she was going to be this heavily armored (even if in leather, and even if people who know what IronStylus does should have expected it). I think that if a side-peek at her helmet or her leggings placed to the side of the journal as she wrote it would have done a lot of work here -- that or use the words "Demacian ranger" somewhere to hammer the point in.
Anyway, this leads some people to conclude that Quinn was "covered up" -- that is, a) she was heavily armored so as to avoid "sexualizing her," and b) that this hurt the clarity of her role and character because the armor is contradictory to her skills. For people who dislike the rhetoric of the anti-sexualization faction in League and other media, this appears as if a paranoia about female sexuality has ruined what would have been a good character.
I don't think Quinn was "covered up". IronStylus explained all of the elements of Quinn's armor and those explanations make sense (pads to kneel for shots, gauntlet for punching). But I think it's worth treating the criticism here fairly. People wanted a tracker/ranger and were surprised when the amount of armor and stylization on Quinn ran counter to their expectations of what trackers and rangers look like. We should respond to these criticisms by pointing out the functionality of that armor for a ranger (as IronStylus did) or showing that the typical ranger elements of fantasy aren't essential or are contrary to League's ethos of what a Demacian ranger would be like (even the mages of Demacia are armored so we shouldn't be surprised that its rangers are).
And second, even if we dismiss the "covered up and therefore not a ranger" thing, some critics still wish she had some more obviously feminine characteristics. This doesn't mean the same thing as sexualized - they just want some more gendered elements.
IronStylus adequately responded to these critics (he wasn't going for the typical ranger, and he was going for androgyny). So he's fine. And I can even point out some great elements that still manage to show this is a woman (the armor accents on the hips, for example).
But you could at least take the criticism seriously. It doesn't help discussion to frame the other side's opinions as buffoonery. Nor do arguments that "Quinn's different therefore good," or "Quinn isn't sexualized therefore she's good" work here. No one is saying that Quinn's armor wouldn't work for some character. The criticism is that for this character -- "ranger" -- it doesn't fit. (I do think it fits, but saying so isn't as simple as "GD all dumb misogynists").
I'm going to refer to some of Magic: the Gathering's concepts of what a ranger is. Some of Magic's rangers are scantily clothed, for sure (sylvan ranger -
http://i.imgur.com/z3Fv6np.jpg). But knowing this is IronStylus, the following is what people might have been expecting.
Frontier Tracker Daybreak Ranger -
http://i.imgur.com/UglySbV.jpg
Those above are closer to typical fantasy rangers. I think the last one especially is cool. But Quinn isn't from typical fantasy. As a commenter below says, we already have elements of typical fantasy rangers in Ashe. We don't need to double up. (And that I think is one of the biggest arguments that defenders of Quinn have). Quinn is a Demacian. This is Magic's idea of a Demacian ranger.
Ranger of Eos -
http://i.imgur.com/oqiSSv1.jpg
I expected something more like Daybreak Ranger out of Quinn. But now that I see what Riot was going for, I appreciate the kind of ranger that Quinn is.