Yes, it is a difference. A full order of magnitude.
Imagine, instead, if the WoW subscription were 150 bucks per month instead of 15. You think people would still play it?
Well, I could more easily imagine a WoW daily pass at 1.50, for example, even if it would end up more at the end of the month, should I play the whole month, because now, if I want to play 1 day, I have to pay 1 month, 2 months if using the pre-paid coupons. And believe me, these days I only charge my WoW sub for when my daughter wants to play a bit. And it "hurts" a bit too, spending 13 euros so that she will get to play maybe 3 or 4 hours over the course of a month. Didn't hurt so much when I spent 3/4/+ hours per day, because I felt I was getting good value for money.
I like HS's model better: if I want to buy 1 pack or 1 arena entry, I don't have to pre-buy a ton (although they sell 40 packs at a discount, but I can either take that or ignore) - I'll buy accordingly to what I feel like / can afford; and if I can't afford with real money, I'll have to make do with gold earned elsewhere (dailies/whatever)
So, you'll only spend 150 bucks on HS per month if you BOTH WANT and HAVE them, so I see no problem with that. But then again, I'm not even playing beta right now, so I can't say if this game will last on me when I get to play it, or if I'll get bored quickly... Currently, I like the fact that I can leave and comeback a day/week/month later and continue where I was, not paying any monthly fees, so if I have a few hours to spare, I can pour them over, but I can also do something with 10~15 mins here, another 10~15 there, which I like.