It's about 2X bigger than the competition, so it will take up way more space on the TV unit.
It has an inferior GPU and memory when compared to it's nearest competitor.
It's more expensive.
The packed in controller is inferior to the competition.
The system doesn't have anywhere near as many exclusives, be it games that are either outright exclusive to that one system or games that you can't get on the rival system.
Fewer and less established 1st party developers, with much less acclaim than the competition.
Smaller install base, thus the online community is way smaller than PS4.
MS doesn't seem to care to sort many of these issues out.
MS are all talk and no action.
No ability to swap out the internal HDD for a new one.
Weaker multiplats than PS4.

The cons seem absolutely clear, personally I've been open to MS showing me games that may interest me, but it's mostly the same old milked stuff, very little fresh ideas and practically nothing new from 1st party.
It doesn't have Horizon Zero Dawn (which has been outright stated to be coming next year), no Uncharted 4, Ratchet & Clank coming, along with amazing things like Dreams.
No No Man's Sky, no Bloodborne, no Tearaway Unfolded, no Driveclub, tonnes more you just can't play on it.
Lack of Sony's quality 1st party studios, that have been baking tonnes of exclusives and thinking about how to step up their game.

Hell even Wii U actually has a far bigger line-up of exclusives and other bonuses, like a cheaper price, smaller box, the Tablet Screen, rechargeable controllers. Nintendo's quality and proven studios.

I'd definitely buy a Wii U before I'd get an XB1 as my 2nd console. As for this nonsense about saying your PS4 has been gathering dust, I don't buy it. People don't spend money on something they have no interest in, especially a console, which quite frankly you could take that money and buy more multiplats for your XB1 or whatever system you game on, just buy more games for your chosen platform.

Just my personal perspective on the matter.

This pretty much hits the nail on the head. 

Also, I'll just say that the Dualshock 4 is completely awesome in every way. Meanwhile, the Xbox one controller is practically the same as the 360 controller and still runs on batteries. 

The major con for me is the library. There simply aren't titles i'm interested in on the machine and its missing that one title that will break me through the door.

2016 is ridiculous for Japanese titles (which i'm mainly interested in) and Xbox simply doesn't have many at all.

What are the pros? It's been out for two years and the UI is demonstrably slower than the Xbox 360 UI. How is this even possible? That alone was enough to stun me after choosing my 360 over my One the other day. The 360 controller is much more sturdy and doesn't feel like it's made from cheap plastic like its successor. How is this possible?

This particular issue boils down to personal preference but my two favorite multiplayer experiences on the 360 (Rock Band and Magic) are watered down versions of their former selves.

With the Xbox One I get non-gaming services shoved in my face. I had to look up online how to access my game library. Accessing your gaming library on a game console should be so crystal clear it shouldn't even need a tutorial.

Oh, and it's ugly. I mean really really REALLY ugly.

Xbox live gold is for 1 account WTF!!!! instead of for the console. (did they change that?!?)

On the 360 every person needs their own gold account but as far as I know on the X1 you only need one gold account per console as long as that gold account is set to share everyone that logs on that console can play online regardless of if they are gold or not.  It more or less says the same thing here so it must still be true.

http://support.xbox.com/en-US/xbox-one/xbox-live/xbox-live-gold-sharing-features

sweet god bless

Cons: I don't own one, my brother does.

For myself and I imagine more than a few others who already own a PS4, the question is actually "what are the pros?" The redundancy is real, the superior performance is noticeable, the fuller Japanese library can't be denied, and...well, your friends are more likely gaming on the more popular system. On a subjective level Sony 1st party output is far more eclectic, which is better for me than "predictable but reliable."

It's not that it's an inherently bad console, the ps4 is just a better deal for the price. You're getting a stronger system for the same price. Hard to argue against that.