Count yourself lucky if you didn't use any of that stuff because it really sucks when they were some of your favorite features of the OS.
There are things I didn't use that other people have lost as well so they are not as much in my mind as well. But whether or not some people use them or not this stuff adds up and starts burning a large number of users.
History grouped all communications to any one person and listed them by date phone calls, voice mail, SMS, each email account, etc. So the way you communicated was not important but who you communicated with was what was important.
The people app used to support notifications for individuals. Whether it be an email, voice mail, text, missed call, etc. You could pin your boss, kids, significant other and get notification and history for just those select people. It was a wonderful feature. Now none of it exists. It was all replaced by round photos that roll around. I get really ticked off when I see all that was stripped out and that photo just rolling around like some stupid ball. Functionality before fluff!
Some of the early builds of W10 mobile contained the legacy people app. It fit in the new OS so nicely too. :'(
Dark theme support was done consistently win WP 8.1. Cortana was done perfectly. Now in WP 10 and Windows 10 desktop her results are always in a white background box and look very disjointed. However, it looks like the new Skype may actually be supporting themes correctly.