So many things that were in 8, are missing in 10 (ability to pin contacts to the home screen, etc). The length of time it takes to get updates pushed to us, most of us won't see improvements unless we buy a new device. I'd update to 10 now but again, I cant see being without many of these things unless I dump $600.
Yes, as shown above, you can pin contacts to the start screen
We've had far more regular updates with Windows 10 Mobile than we ever had with Windows Phone 8.x
With Windows Phone 8.x, you'd need a whole OS upgrade to upgrade Office Apps, Messaging, and the other core apps. These days, they're just App updates that happen so frequently that I tire of reading about them on the Windows Central front page.
OS Updates for Windows Phone 8.x used to involve months of testing with different carrier/region variations. It's immeasurably faster with Windows Phone 10. It's impossible to measure how slowly updates for existing devices will be until updates are announced, so I don't see how you're using that as an argument here.
If these are you biggest concerns, you should be really really happy.