The reason why Twitter is successful is that its so simple, there are millions of use cases.
If Thingist could automatically categorize my updates, but have a way for me to re-organize them, that'd be ideal. Seems like a decent amount of people fall into your same use case, where they tweet quotes, songs, videos, etc. Some of these would be easier to detect than others, but it's somewhat doable.
This seems some like weird cross section between Twitter, Tumblr, and delicious to me.
Categorizing them is really, really hard to do. Look at the ways that this user uses the site: http://thingist.com/t/item/3051/
"Things that make me feel better when I'm sad" would be impossible to detect programatically.
"Show me all of the links to youtube or soundcloud I have posted" is totally doable, and is something that will probably happen within the next couple of days.
That said, people are willing to fill out extra information about (look at digg, reddit, youtube, fark, etc. etc.)...maybe not for status updates, but thingist isn't about status updates.
Use cases for twitter? I get hashtagging events (I saw this first hand at the intel appup conference last week), but what else beyond that?