I've been through two Thunderbolts so far - first one had three dead pixels, second one had annoying creaking coming from the frame around the screen (left side, right where your thumb would be if you're holding the phone for a call). All replaced with no hassle.
Now this third one has a creak when you press the power button down, and creaks in the same place as the last one, on the frame where my thumb goes for a call. I've never taken the battery cover off, and it seems to be in place well. Sounds like it's coming from the frame itself. The models in the store didn't have this; my friend's iPhone 4 is rock-solid (just saying!). I've already ordered a case, so I figure I'll see if the case covers up the creaks so I can avoid looking like 'that guy' who's way too nit picky and taking back my phone a third time.
Not trying to derail thread, but I'm just throwing that out there. If it bothers you, you might be able to take it back and get it covered. I do love this phone - awesome screen, and Android is leaps and bounds more fun than iOS. So I'll just chalk it up to being a new LTE phone.