Probably the main reason it hasn't completely dominated is because MS is taking a weird partner-oriented approach to VoIP with it. So getting setup with some numbers and outbound calling is tons more work than it should be.
Skype is good too but be careful if you use it for IM. I frequently receive messages hours after the other person sent them, even if we're both online. This is starting to make me not want to rely on it other than for voice/video.
Pros:
-Outlook Integration
-Screen Sharing
-Decent UI
Cons:
-Literally the worst copy & paste implementation I've ever seen.
-No third party chat protocol integration. (I want to integrate Facebook and Google's Jabber).
Edit: Formatting.
What XMPP server are you using or would you use?
A stupid configuration prepends an underscore to urls, preventing it to be clickable. Hate it.
While I've not seen any use to adopt G+ in a public way, it's been a phenomenal internal comms tool for async sharing, discussion and debate that isn't critical enough to go on an everyone@ email list.
Pros: integration with PT and Github rules; mobile client is better than decent.
Cons: transcripts/history can be wonky; no way to add metadata to file uploads; native OS X client seems to randomly eat characters when typing.
Still pretty new, and in closed beta. But it adds an interesting threading/topic model onto the typically chaotic stream of most group chat apps.
And sometimes XMPP.
My setup is LimeChat -> Bitlbee -> GTalk.