I've been on Android for 6+ years, and decided to see what all the fuss was about with the iPhone so I pre-ordered the 6s 64GB. The phone is designed beautifully and feels great in hand. The display, while small, is fantastic.
My issues are with the OS itself and just some oddities in general.
App navigation is horribly inconsistent. I find myself having to swipe down to go back to the previous page, then swipe left to go to the menu. I'm always swiping in different directions as each app seems to have their own preference which way to get back to where you were. Snapchat and Google Play Music are two that I use a lot which cause lots of confusion.
Duplicate notifications! When people like my Instagram photos or Snapchat me, my lockscreen and notification shade are filling up with tons of individual notifications like "such and such liked your photo", or "bob is typing". Do I really need to see a notification for when someone was typing to me in snapchat and when they sent it? Instagram is the worst where it lists them all out individually. Android is far better in this regard where it will say for example "Bob liked your photo" with a 25 in the corner letting you know how many people liked it, basically it combines all similar notifications together so you don't have to scroll through them all.
Notifications don't go away on their own. If I get a notification, why does it remain if I open the app manually and check it? If I get an email, read/delete it, then open my notification shade it's still there.
The camera won't remember my settings. I want HDR to be on all the time, why do I need to enable it manually every single time I open it up? And why can't I set photos to use 16:9 instead of 4:3?
You can't save a simple photo. I'm used to Android where you can have a photo in Dropbox, Email, a website, anywhere and just long press and save/download it. On iOS the only way I found to get a photo from a computer to my phone without plugging in is to upload it to the iCloud website and then connect to WiFi so it will sync.
No back button kills me. I hate having to figure out which way to swipe to go back, or hit the top left of the screen. So much easier especially when one handed to hit the back button next to home.
I can't change Mail to change from archive to delete. I just want to delete emails I don't need dammit!
The homescreen layout is so cluttered with useless apps that Apple even has an "extra" folder by default. Either let us hide/disable apps we don't need, or change the way apps are displayed. Also, why can't I simply organize apps on all other pages alphabetically? I hate how it puts them in by default in order of when they were installed, it's so much easier to find apps on Android when everything can be sorted.