Looks over usability is poor business move, even if 30% of your users are having issues using the device you need to solve the issue by changing or offering a choice. But in this case I think they made the right move, in fact, if you had never seen the first ultra thin font, you would be very happy with the current bolder one.
It's still very thin compared to ios6. looks great, and most likely here to stay.
My vision up close is great, just at arms length...thin text, especially black on white tends to be hard to read, and has a odd blurring effect, and not easy on my eyes, black on a darker shade is OK. Also holding the phone very close was fine with the old font, but I often glance at my phone at a distance for a quick look at info, ios6 is still better for that, especially in the weather app, but I turned on bold and it's not bad.
So it could be the combination of so much white and the thin fonts that bother some people.