I also thought about this when I started building Testimote. A lot of interesting things are being said (positive things about companies in my case), and are drowning out there in the great torrent coming from the firehose. I think there's room for both Twitter and outside companies to continue to build out curation ideas around this, as the data set is quite rich (and Twitter - at least recently - has been fairly generous with their basic APIs with things like rate limits [though I think lower cost plans would be fantastic for higher rate limits, rather than big money for the full hose].)
A combination of better curation and rebuilt developer relations could really help - maybe not user growth, per se, but certainly with service usage. What about web sites and mobile apps giving people a way to give feedback to the company (something as simple as a Tweet box that says "give instant public feedback") that simply tweets out the message using their account? People would start using Twitter without doing anything more than filling in a form and supplying an email address (for first-time users.) There's a lot of untapped potential around this. I think it also helps to think of Twitter as a giant, sometimes overlapping set of IRC channels and DMs, but with the ability to tap into it all. Like with IRC bots that would catch stuff, and put it in a database. Interesting conversations happening, not much being done with it. Twitter could encourage people to do more with it from the outside, and help everyone, including themselves.
It still has tons of potential if cards are played right.