Five points are deducted from Gryffindor for restarting your numbers.

1) You follow people, tags, or domains. You may also BLOCK people, tags or domains. For example, you could follow thenewgreen but block theatlantic.com because it's become mostly empty posturing but lots of people haven't figured that out yet. That way, you could get all of TNG's insightful articles minus the bilgewater from The Atlantic.

2) Posts need not necessarily be tagged. However, anyone with a modicum of participation can tag any post. And they can tag it anything. Which leads to a lot of passive-aggressive ironic tagging. Which is a problem with tagging, but not one likely to get sorted today.

(Un-numbered) The interface was borne of Hacker News but has been rewritten since.

(Un-numbered) Yes, twitter also has tags. However, because twitter is fundamentally devoid of content, the "interest algebra" possible by following and filtering users, tags and domains gives you a much more complex and nuanced feed.

(1 again) your feed is comprised of the things you follow, minus the things you filter. I'm hesitant to elaborate beyond that because mk tweaks it occasionally and I'm not sure what the current hierarchy is.

(2 again) I'm not even sure titles are limited to 140 characters. The limit used to be shorter than I like (because I often use quotes as my post titles) but I haven't had that problem in years.

(3) comments and replies become a part of an active conversation that can go on for days weeks months or years. Hubski does not archive; it's possible to reactivate old content from weeks ago and have it appear on a user's page.

(4) there's also a robust PM system which many people use more than posts and comments. PMs are not necessarily person-to-person; they thread and concatenate just like comments. However, there's no hubwheel functionality on PMs and you may not see all responses if you don't view the top PM in a thread.