I was just thinking the other day how I miss MSN Messenger. Toward the end it became bloated and awful in an effort to include more crap, but the DNA of Xbox Live can be seen in the first inclusions of Bejeweled and Hexit.
Like Facebook after it, I was mostly roped into MSN Messenger by friends who were using it, and like Facebook after it, it didn't take long for my friends list to fill with people I barely talked to and wanted to avoid. For years, the only thing keeping me from using Google Talk was text formatting. I'd grown attached to my particular font colour (made possible by Messenger Plus, actually, it wasn't one of the default sixteen colours), and my bold, italic Times New Roman typeface. Further to that, in multi-person conversations, everyone's individual font style was essential to identifying who was talking at a glance. Hangouts just doesn't offer that.
Another good thing about MSN Messenger: It was an IM application. They added offline messaging, but it was always clear who was online, who wasn't, who was online but away, who was busy. Google Talk used to do that, Hangouts is terrible at it. Sure, there's a green line under a profile picture for people who're active at their computer, but you don't see that on mobile. MSN Messenger was made for talking to people who were there to talk back, and for shutting it off to make plain you weren't available to talk. Hangouts doesn't do this well at all.
The file transfer feature of MSN Messenger was useful as all hell, until they started trying to make it more "secure" by blocking .zip, .rar and .mp3 files. Funny how they didn't block the real Windows security risk, MS Office documents.