Sync tabs between devices.
The ability to have your tabs and synced between all devices that share an account so you can move from one device to another without having to find all your spots again.
I currently use 2 operating systems, Windows 10 on my tablet and iOS on my phone. I actually enjoy not having the two synced with the tabs. It makes me more inclined to use the Bookmark feature, which does sync across both operating systems. It also makes the experience a lot more distinct for the two.
Samuel Bradshaw commented
Whichever way it goes (syncing or not), this is a decision that would need to be made together with iOS and Android, so behavior is the same across devices.