I'm just going to go through my phone and randomly pick out things I like and hate. Also, be warned now, I really don't like Samsung.
I don't have the M8 yet but I can tell you right now that Sense OS is the best stock OS out of all others. Samsung touchwiz is a joke, bunch of bloat, and I hate how they would love to go against Google's design philosophies (Anyone who says Samsung's touchwiz is better has buyers Stockholm syndrome.). LG and Sony are ugly as well, just too much going on, even unlocking an LG phone is cartoonish and silly. Sense is seamless, smooth, everything is where it needs to be. This is probably my biggest selling point if you plan on staying stock.
The screen is the best out of most of its competitors, it's great and has decent outside viewing abilities. AMOLED is also great if you don't ever see the sun, so if you're a recluse and live outside of direct sunlight it's perfect.
Metal body is great. Yeah, it is important. All plastic phones have ugly faces, at least I can look at the speaker grills on my One and feel like I'm not holding onto a toy.
Boomsound. Self explanatory.
The camera on the One is horrid. Poor excuses from HTC to be honest.
Maybe HTC should also learn to nanocoat their phones like Sony does so it can be waterproof? I don't know if it would work or not, but that bugs me too.
You don't have to worry about HTC trying to region lock your phone like Samsung does. I travel every year so this is just a no-no.
Best front facing camera from pretty much all of the immediate contenders (this is great if you're a white teenage girl).
M8 is too tall thanks to that dumb black bezel.
Some multimedia stuff that Habiib said that I personally couldn't care less about (expect maybe the TV thing).
What else is there to a phone anyway? Aside from Samsung gimmicky software that barely works (that eye stuff was so cool until I went into the store and it went crazy on me), the phone can do everything else its competitors can.