@sonyam pump.io has private messaging, followers-only posting, federated identity so you can run around on other servers and comment using your home id, the API is lightweight and very consistent and understandable in comparison with GS+Qvitter (I hear), and it's the basis for the Next Big Thing, #activitypub.

pump.io was built in a few months by one guy though, and GS was built in a couple of years by several full-time people, so there are still quirks in the (actually intentionally) limited web UI. pump.io works best with a client.

@andstatus does a decent job. I'm not going to complain, because I still didn't file those bugs since last I complained, so the buck is with me. It's mostly about message formatting, other than that it's an awesome client for both GS and pump.