Just to be clear, I was only commenting that GMail's search
can be used for chats. I was not commenting on other features, but I'll try to answer your questions if I can.
> Does hangouts have a concept of topics?
Yes, in that you can title group conversations. It's weak though.
> From my limited uses of it, if you chat with a single person, you can't create a new chat with that person around a different topic.
AFAICT, this is correct. If you try to make a group chat with only one person, it will just open the existing chat that you have open with that person.
> In hangouts, how do I easily see the files that have been sent?
I honestly have no idea. whenever I've needed to send a file to someone over hangouts, I share a dropbox link.
> Also, how do I star a comment? In slack, it's handy to star a comment, or even pin them, to make finding them easier.
This is not possible AFAIK.