From talking to myself (wish I could find some people to test this thing with), the features aren’t that interesting. It just feels like texting on a desktop.
The reaction GIFs that are straight up pulled from random Tumblr blogs aren’t that neat either, at least when being used in a conversation. 
Heck, the emoticons are just the generic Unicode emoticons:
As per usual, you have no status indicator, no buddy list (instead, its treated like a texting contact list, which shows your recent convos instead, which I see as unnecessary and which can get pretty discorded after a while), no buzzing or anything to get your recipient’s attention (including Audibles). and the most bland and uninteresting UI I’ve ever seen. Heck, Discord/Slack at least try to pep it up.
There are @mentions in the new Yahoo! Messenger, but I don’t have the right setup to properly test it out. Unfortunately, this isn’t a viable replacement for buzzing, as the @mention will be sent via notification, which can be disabled, along with the fact that it will basically take up the chatting space.
The slightly interesting thing I’ve noticed in the UI, though, is a “Get the app” button. When clicked on, it takes you to the page where you can download Yahoo! Messenger for iOS/Android. I can pretty much deduce that Yahoo!'s trying to appeal to the texting market, which isn’t my cup of tea.
Overall, I wouldn’t really use the new Yahoo! Messenger. At all. It’s not interesting enough to keep me in, and it’s so simplistic that I would rather use MSN 1.0 or AIM instead.