Gonna post an opinion contrary to some others: Window Shoppers in particular are cute and okay, but nothing more, and the DH class in general could even use some buffs: for example, that Ci’Cigi is more of an insult to the class (yeah, get some cards from back when your class was somewhat good… which isn’t even that great now, especially the Metamorphosis) than a ‘proper’ legendary (maybe at least change the effect to a battlecry?), Magtheridon looks decent only when discovered with the discount, and so on.
I’ve played with that deck and against it (written a bit about my initial impressions), and while I’m not the most seasoned Standard player at the moment, I believe I’ve seen enough to form an opinion at least at some level. What some of you seem to be overlooking is that this deck isn’t just about somewhat discounted — more about it below — minions, which could be removed, with the exception of random Magtheridons — it’s about hitting the opponent’s face with your hero as a source of continuous damage, while also maintaining the tempo. I mean, come on, what’s new or unusual with that? Ask those face hunters from 2014… It’s also quite clear in general how to counter strategies such as this — it’s not like this is some unbetable Exodia or whatever.
Yep, it’s one of the more aggressive decks, but what of it? And who are the main victims of this kind of archetype? Some ‘cheesy’, but slow or vulnerable decks? Think of some examples: ‘Oh, I’m gonna be sitting there, feeling my, ahem, camisole (unnerf Jaina, btw!), doing mostly nothing, and then, after umpteen turns, you suddenly die, because I’m a Sif/Pow-Pow Mage or something like it, just don’t you dare touch me before that, because I’m playing my own rainbow solitaire’; ‘Barrels of laughs sludge, lol, and then you just die, wheels or whatnot’; ‘Me take extra candle turn’ — or, apparently, not even that, according to some reports? If there’s no reasonable aggro/tempo/etc in the ‘meta’, then that’s what you’re gonna be playing against — GLHF with that, just remebmer the cautionary tale of the Patron Warrior, for example: while that deck was… controversial, unlike the subject (Window Shoppers don’t OTK you from 50 health or something like that, you know), its disappearance made the meta arguably even worse. Besides, I’ve got no issue with Aggro vs Control vs Midrange and so on, it’s uninteractive OTK-ish stuff and such that is problematic, as well as some ‘disruptive’ effects (gotta make another topic about it, perhaps; UPD: The game needs more 'disruption', OTKs and solitaires).
As for the stats and cost in particular — yeah… is it surprising for modern Hearthstone, where even a 4-mana 7-7 minion is ‘meh’? A 3-mana 5-6 — that’s like a (buffed) Worgen Greaser (the poor guy didn’t even make it to the Core, where no-one would probably play him competitively anyway — and his rivals for inclusion in the set were the Chillwind Yeti and the Boulderfist Ogre, who were once considered to have good stats for the cost… Oh, 6-5 is OP compared to 4-5 for the same 4 mana!) with a bit of a discount, and in order to get it, you’ve gotta play a 3-mana Fiery War Axe. Well, yeah, a Fiery War Axe followed by a Worgen Greaser is too OP? 
Gotta start a topic about it some time, perhaps (UPD: Worgen Greaser is OP). But, as I was saying, are we seriously gonna complain about 3-mana 6-5s and not, say, 1-mana Sunset Volleys coming in… volleys?
Speaking of which, I had a particular game with a Plague Death Knight against a Clown Tentacle Warrior… Warning: all of the following is to be taken very seriously, no humour at all intended. 
So yeah, that undoubtedly talented player dodged a bunch of plagues like a pro, got his Deepminer Brann going before I even had a chance to draw Helya, then proceeded with exceptional RNG without a volley of Sunset Volleys without a single Table Flip. This single game is clearly a sign that the aforementioned card is an unstoppable Exodia-like one, guaranteed to come into play at 6 mana even against its direct counters ans instantly destroy the opponent’s hero, at the very least morally . I’m astonished the forums are not rife with complaints and suggestions to nerf it at least to 15 mana, which isn’t really even that hard to pull off (Audio Amplifier and a couple of coin pouches, for example — hey, you’ve gotta do at least some work to summon Exodia).
Anyway, as for the Window Shopper… I’d suggest the following fix: ‘If you would draw this card normally without a discount, shuffle it into your deck and draw another one instead’. Hard to formulate a concise keyword for it, though.