The OS doesn't make it a desktop. You still have to purchase the keyboard in order to consider it a laptop, exactly like an iPad Pro. Neither tablet comes with an attachable keyboard.
I have to disagree.
Among the things I can do with a "desktop" OS that I cannot do in iOS.
I can save files where I want them to go, iOS gets around this a tiny bit with the iCloud drive but for intents and purposes the file system isn't exposed to the consumer.
I can easily attach files to my email client.
Applications are more powerful on a "desktop" os. Just look at what you can and cannot do in MS Office for iOS
I have full multitasking,
I have multple windows open
I can use a mouse
I can hook up an external monitor
I can hook up a USB external drive,
I don't need to rely on a split screen setup to run more then one app
Sure I had to buy the keyboard separately and I do think that's a mistake on Microsoft's part, because the major useage of the product is for a laptop replacement.
iOS is a good product, but SP running a desktop OS (or let me say desktop/laptop OS) is much more extensible, powerful and flexible then the iPad. Does that mean the iPad poor product, no, but for me I want those features that the SP offers.