I am one of the fortunate MANY that has a loose power button on my M8. While I'm not worried about it falling out, the slight rattle it causes when putting the phone to my ear is enough to make me want to jump off a bridge. Mine is only 2 weeks old and just started. However, the stock at my local VW store had the same issue right out of the box. Rather than squabble with them or try to get another phone through HTC and have to deal with wiping this one and setting a new one up, I have a quick fix.
The button just needs something to hold it in place without putting pressure on it. SCOTCH TAPE. Put a small sliver of scotch tape across the button to hold it. A case will keep it in place, you should have one on this beauty anyways! It's all but invisible even in my clear case. Sometimes the simplest fixes work best.