iMessages
Calendar - I run a business and if I make an entry in my mobile calendar it is on the desktop, I don't need to think about not and I don't really need to worry about it.
Contacts - see above
Music - I use a few different music services but when I build a playlist in iTunes on my iMac I don't have to think about it and it is available on my phone, my iPad, my macbook
Integration. I don't have to worry about a third party app to sync through iCloud - I make an entry in notes/calendar/contacts/etc and it is everywhere I need/want it to be.
I can answer my phone on my MBP/iMac if I want to - seamless integration...in the end it is all about seamless integration for me. I have owned and still own a couple of android products from different manufactures and they all have on thing in common that I can say that I have never thought about an iOS device (and I know some will disagree with me) but I have never thought when using my iDevice that it doesn't feel finished. In my experience with an android device it feels unfinished, unpolished...I don't need or want that, I don't want to be the end piece of the experience because I just want it to do what I need it to do.
I tell people all of the time to choose what works for you, sure I am jealous of the gorgeous display that they Samsung has but at the end of the day I will choose an iOS device because it is what works for me. I don't have to think, I don't have to worry like I did back in the palm pilot/treo days...did I sync last night, it is already done.
Yes I can accomplish all of this integration using various android apps but I don't have to...it is all baked inside and it is everywhere I need it to be. Watch, phone, tablet, desktop, laptop - all there, no need to think about it.
And as Love Devine has listed - NO BLOATWARE - you get iOS and that is it, no ATT/Verizon/Sprint/etc fiddling...updates come to you through Apple not after a 'tech team' gets through testing and customizing it for your network.