The new Google Hangouts in Gmail option seems interesting, and while it shows my most recent contacts, most of them aren't online (some never are because of the Google account they use). Can I sort them by their online state, as the old chat does?
No. A Google employee I know told me the following: "the idea is to free you from worrying if the person you are talking to is on mobile, on a tablet, in front of a computer, etc, and instead give you one channel by which to communicate."
For those of us who like to know exactly which friends are on mobile (or even online at all) this is a big step backward, but it is the direction Google wants us to go.
Your best bet is to run a multi-IM client like Pidgin or EBuddy, which has more flexible display options, and switch to Hangouts when your friends are ready to video chat.
At this time, it is possible to see who is online in Hangouts. From See people’s online status - Hangouts Help
When you want to start a Hangout with someone on your computer, you can look at the person's profile picture in your Hangouts list to see whether they're online and available or not. If they're not online right now and they have the last seen feature turned on, you can see when they were last on Hangouts.
Search for and add contacts to your Hangouts list. You can also manage your contacts list to pin specific people to the top of the list or hide others.