Well, I had a go at describing the changes from Threshold to Redstone here on AAWP, in long form and with commentary. But sometimes you can't beat a good old fashioned bullet list - especially when it's as thorough as the one compiled in the linked page by the ChangeWindows team.

The full user-curated, but seemingly comprehensive, list is too long to quote in full, but here's a sample:

Cortana and search

• Cortana is now available in Spanish (Mexico), Portuguese (Brazil) and French (Canada)
• Cortana will now show a button on the top to search for music
• Cortana can now provide turn-by-turn directions with the new Maps app
• Cortana can now sync across devices
  • You'll now get a notification whenever your mobile device is running out of battery
  • Find my phone/Ring my phone is now available through Cortana
  • You can now share map directions between your devices
• Cortana can now show new notifications in the Action Center
• Cortana can now automatically fetch the required speech language for you
• Cortana can now give reminder suggestions
• You can now change Cortana's language
• Cortana's Settings have been moved outside the Notebook
• You can now create reminders with photos or shared info from apps
• Improved share UI for reminders
• Cortana can now search in your Office 365 account
• You can now send a picture from your phone to your PC through Cortana
• Notifications on your phone will now be synced to your PC with Cortana
• Cortanas listening animation has been updated
• Cortana should now listen more reliably after tapping the mic button
• Cortana's home UI no longer shows your cards but hints and a button to show the cards

User Interface

• The back-button on the lock screen has been replaced with a camera button
• The lock screen now shows media controls directly on the UI
• The Bing logo on the Bing lock screen has been updated, appears smaller and no longer has the text "Bing"
• The data migration process now uses a larger font
• The "Goodbye" message will now stay visible until the device has been shut down completely
• Glance screen now follows the Ease of Access scaling settings

See the full changelog here.

As I said in my original analysis though, there's no real question of anyone on a W10M-approved device not updating, since it will appear automatically. Still, good to have something at least partly definitive to refer back to.

And yes, if you want something that's less definitive but more readable, see here.