I value communities too, but the open broadcast nature of Twitter is what I like about it: I can put a message out into the either ("sector-level FDE encryption is bad!" or "ChiSec is next Wednesday") and it quickly percolates to people who (a) I'm glad saw it and (b) I would not have known to send it to. Lots of good things have come from that dynamic.
Twitter is full of abuse and harassment. Other communities are, too. One solution to that problem is policing, which is what HN does. Another is (for lack of a better term) cliquishness, which is what closed communities do. I see a lot of value in both of those approaches too, but every community management strategy has tradeoffs, and serendipity is an awfully powerful force to trade for orderliness.
Twitter is full of abuse and harassment. Other communities are, too. One solution to that problem is policing, which is what HN does. Another is (for lack of a better term) cliquishness, which is what closed communities do. I see a lot of value in both of those approaches too, but every community management strategy has tradeoffs, and serendipity is an awfully powerful force to trade for orderliness.