The UI is great, the file sharing is great... But the only major blocking issue right now is the complete lack of an API.
Look, Ruby is great, but when you are integrating a large project, with existing chat services, you need a standard protocol with good API libraries. Right now on our IRC channel, we have Bots written in Ruby, Python, TCL, and C/Lua.
They perform all kinds of services, from searching Google, to SVN commit notification, to controlling our buildbot builds.
If Campfire at least had some kind of official API, it could be XML based, JSON based, or even something else, I don't really care -- it would allow us to easily port our existing tools in many languages. Heck, if it had an API, you could also make a Jabber Gateway easily.
So, here is my request to 37 signals: I just want an API.
An API so we can become entrenched into your solution. So we would never leave. Right now its a hard sell -- we would be either writing/porting Tinder for 3 or more languages, or investing lots of time in a Ruby based Proxy of some kind, but that all seems excessive. Look at the other 37 signals apps with APIs, a whole ecosystem of things that support your products appear. APIs are good for you, and good for us, a potential user.