Twitter should copy Google+’s circle feature.
geekybuddha.org
geekybuddha.org
Also, a primary difference between Facebook (now Google+) and Twitter is that Twitter was assumed to be public by default. It is a public form of communication, and the Google+ style of sorting your messages into specific buckets of people doesn't fit it at all.
These people then end up having multiple Twitter accounts, which is a drastically more difficult "extra step" to perform when they go to post a new message: sufficiently so that a good number (most, in fact) of the people I know who do this simply don't bother, and /only/ keep the private account.
(Although, I personally am in this overall category, but chose to keep only the public one, and thereby simply "don't really use Twitter, as they apparently only care about weird totally-public PR garbage".)
If Twitter added the circles feature tomorrow, people who actually considered it a pain to think about simply wouldn't: they would ignore the box, and business would go on as usual. However, if they wanted the feature, they would probably start using Twitter a lot more; and, as they are no longer "rapidly" switching accounts before posting, that allocated time would be even more effective.
(Also, for the record, if Twitter did do this, it would be even extra awesome, as "listing" someone already provides all of the "circled" data model, meaning that a lot of people, like myself, for whom this feature would have immediate value, already have our followees organized usefully.)
Because all modern cell phones have Wi-Fi, and there are a lot of free hotspots around the city (here in Mexico), they use twitter as SMS... They log and send a message to their friends about the hour to hang out, and the place, and the chat over twitter... I'm nor sure if they know it's public, or the implications of that, but they truly do that...
I'm begging to understand twitter, as poor man's phone calls...
I have zero interest in managing and maintaining circles, and have already given up on doing it in Google+.
What Twitter needs to build is a capability where I can advertise different topics that I talk about, and then people can selectively follow everything I tweet, or just follow tweets that I publish to a particular topic. In G+ terminology it would be like if they made my list of circles public, and then people added themselves to circles rather than me adding them.
I don't think a naive keyword search would really solve the problem. I did consider at one point writing a Twitter client that would do simple Bayesian filtering across a list. So you would create a list, add whoever you want, and then flag messages as good/bad for that list, and it would start filtering.
Twitter's simplicity is at it's core, however. I don't think Twitter could slip a system like this in very easily... nor can facebook.
With Google+, the circles are core, not just some tack-on optional feature. The main way to share something on Google+ is by sharing it with circles.
To put it another way, the default on Facebook is to share with all "friends". The default on Google+ is to share with nobody - but the list of options starts with circles, and ends with the public. (Google+ also lets you include individuals in the list, but you have to type their name or email address, so I suspect that feature is little-used.)
I swear Facebook used to give you the option to view your incoming feed filtered by friend list but it's not there now. Maybe that can make a comeback.
That's not the point, however - it's how it's presented to prospective users and how the UI works.