Re: Plain and simple: Reasons for me to buy this phone.
It's the best Android phone available for me personally. I don't like phones over 5 inches. They're too wide to type on comfortably. It feels so good to hold. It's very comfortable to use, and my thumb doesn't get tired using for long periods.
It's the smoothest running Android phone I've seen (along with the M8).
Even if the external speakers aren't your cup of tea, the sound coming through headphones is also better than other phones.
The display may not be QHD, but it looks good, and readable outdoors.
If you mostly shoot day photos (even indoors) the pics are pretty good. If you do alot of video or night shooting beware for an uphill battle.
Those are a few of the reasons the One series are the only Android phones I'll use right now. No idea if those are important reasons for you though.