I know too much about it, that I can't say exactly why it's "special", sorry. Most of it would just have to do with internal organizational details.
The below is my take on Hangouts based on it's history here: https://en.wikipedia.org/wiki/Google_Hangouts
It's interesting to remember that it was born out eating multiple existing services (GChat/GoogleTalk, Huddle, and some other things probably). When it launched, it was trying to cleanup Google's fragmented text/video messaging strategy.
One funny thing is how message history search worked. There is no in-app searching history. You can search on the Gmail desktop client and see history, but no where else (no mobile app. Not in Inbox. No-where but gmail desktop).
Along the way they added SMS support for it, which they later scrapped for everyone except Project Fi/Voice customers.
Hangouts has done a lot in the last 5.5 years (since its launch). It may have tried to do too much, which may have caused some problems (especially on usability).
Hangouts Chat and Meet seem to try to start fresh and build a good product focused on business customers. How that plays out for normal gmail consumers has yet to be seen; I'm really hoping they do a good job with it.