Cheap ("Value") Engineering
I've had the same problem. I got a Droid 4 in November 2012, and after only a couple months (of careful usage with my phone in a case and a screen protector, etc.), the power button went all wiggly. I decided to claim warranty service in spring 2013 before it became unusable, but the replacement they sent me was even worse! I sent the factory replacement back with a letter explaining what was wrong with it (that I don't think was read at all) and decided to just deal with it and hope it lasts two or three years, until it's time for a new phone. After only a few months, the button finally got bad enough to claim warranty service (again), but the replacement they sent me is just as bad. The phone's supposed to go through a 100-point inspection, but I guess an originally functioning power button isn't on their list. Motorola's tried to cut costs by using less plastic and fewer parts (a process called value analysis or value engineering), but the power button is kind of a big damn deal. And come to think of it, my two Motorola RAZRs and RAZR 2s I had before this began having cheap manufacturing problems too. (Not the Droid RAZR, the old ones from the mid-'00s). I'm pretty sure my next phone won't be a Motorola.