The new Calendar has usability problems as well and no longer auto-completes addresses in the location field for Cortana to track.
The new email program does have some improvements like lots of HTML email formatting capabilities, but it loses the easy of use for switching between sorting options. If you’ve got a phone with Glance mode, that will not longer support lock screen apps like weather. That’s a pretty big deal breaker since task notifications won’t fire either. The photos app will no longer integrate with Facebook, Twitter, Flickr, and Google Photos. You will no longer have access to Office 365/Exchange Server tasks. You’ll also lose some great features of Windows Phone 8.1. This is a pretty obvious usability failure, yet somehow all smartphone operating systems make the same mistake these days and Microsoft has gone along with the terrible design of the other platforms. Furthermore, many buttons are now at the top of the screen where you can’t reach them with your fingers while holding the phone in one hand. It’s a lot more like Android in that sense. There’s no longer a consistent way of finding commands and finding out what the commands mean by reading their text labels. Microsoft has switched to a more “anything goes” style of user interface design that makes everything much less intuitive and much more confusing. In Windows 10 Mobile, many of the user interface elements have changed for the worse. That is how things should normally go, but that’s not often the case with Windows Mobile. Many people think the newer version of an operating system is going to be better than the current version.
There’s a few things you might want to be wary of when that Windows 10 Mobile upgrade comes along though.
Most people probably have the automatic updates option turned on which will probably notify you when an update is ready to install and allow you to proceed. The Windows 10 Mobile upgrade for existing Windows Phone 8.1 devices has started to show up.