I'm not interested in getting into a pissing match with those with superior knowledge, but having been through three 920s in the last 4 weeks, here's what I observed.
My original 920 that I'd owned for the last 8-9 months... I wanted to reset it to get out from the Xbox Music/Zune mess I was having with my borked music collection (that'a whole other story). The advice from the guys at my local Microsoft store - reset your phone so you can start fresh. So I plug in my phone to my PC, move all my photos and videos, and delete any music files on the device. Left all the apps there...the WP8 desktop software indicates I have about 75% of total available space free. My goal was to empty the phone as much as possible. Back in the phone, in the settings menu, I go to backup and make sure my app list, setting and texts are backed up. Then I go down to About and hit reset my phone. As I said, I had done this several times before with my WP7 phones (Focus and Lumia 900), so the process wasn't unfamiliar. Of course, this was the first time I was doing it on WP8. So I hit the spinning gears - I plugged in the phone and went to bed. 5-6 hours later, still spinning gears, and I'm thinking WTF? Before touching it, I head over to Nokia forums - people complaining about spinning gears and bricked 920s. I come over to WPCentral forums, and again, 920s getting bricked after resets. I tried some of the key combination suggestions - no dice as the phone continues to come back to the gears. Called AT&T, the warranty department concurs (we're seeing this a lot with 920s) and agrees to send me a new one.
So I get my next 920. This time I fully restore the phone with my MS account, reloading all my apps, but I don't have any music or pictures. I didn't exactly check it, but I figure I had about the same amount of used/free space as my original 920 - remember, I deleted all my music and photos from my original phone before doing the reset, so I only had apps. I wanted to see if I could reset this 920 before fully "moving back in" so to speak. I hit reset... and in less than 10 minutes, the reset is done. Not 5 to 6 hours, not 24 hours. But shortly after getting this phone, I discovered that something was rattling around inside the phone. So I called AT&T again and requested a replacement.
So, now I have my 3rd 920. Again, I reload all my apps and MS account configurations. Again, the first thing I wanted to test was the reset - so I reset the phone again right away. Like phone #2, this one came back from the reset in about 10 minutes.
Now I'm just a plain old simple user. But I can't imagine what would make phone #1 so different from #2 and #3 - same phone, same apps, same account, same contacts, same emails, etc...what would make #2 and #3 reset in 10 minutes but phone #1 not finish in 6 hours. Call me ignorant, but I just don't get it.
BTW, my #3 lumia 920...I discovered that the camera on this device won't focus. Yes, it won't focus. All pictures are blurry. Call me a simpleton user again, but I figured since I could take clear pictures with 920 #1 and #2, I figured that something's wrong with #3. Again call me a simpleton, but I figured that maybe Nokia has some quality problems with rattling phones, bad camera optics, and faulty hardware that don't come back from factory resets...
So again, AT&T is sending me a new phone. This time they're giving me a 1020. The warranty guy tells me they're not getting ANY warranty returns off the 1020 but lots of issues with the 920. Of course they probably sold a lot more 920s than 1020s, but still, they're replacing a lot of 920s. So maybe, just maybe, Nokia is having some issues with this device.
Again, just the observations from a simple user, not so all knowingly blessed.