You mean how you can't see who is online on Hangouts? That is super annoying. I may decide how to contact someone based on their status. Are they idle/offline/busy?
most people don't use status and most of those who do abuse them (always away or busy or invisible etc) so I've always found them unreliable and useless. If you need to contact someone, just send the message, he'll answer when he can.
Also statuses (so not only on/off presence) and ability to log off (not only suppress notifications).
Lack of those make hangouts unusable for me.
you can log off actually, it's buried deep within the hangouts settings menu. still supremely inconvenient though
it really broke google talk/hangouts on mobile. it isn't any better in the browser, either.