I think I got here from programming.reddit, saw some interesting links, and added the RSS feed to GReader.
The connection from a community I already know is more important than the self-advertised topic of interest for a news aggregator site. I trust reddit and my homies on del.icio.us more than some site that claims to be all about some general topic of interest like startups. If such a site has something worthwhile, someone on reddit or del.icio.us will probably link it anyway.
I suppose for others, they could have a similar feeling about slashdot, or digg, or something else. At some point, I think users just get comfortable with a reasonable amount of signal to noise on a site and stick to it.
As others have pointed out, the UI is ornery at Startupping. There's just a lot of random crap on the front page...some crap down the middle, some crap on the right side, oh, there's some more crap down the right side.
If it were my site, I'd:
Kill the big posted on date box beside every item--the way it is now, the date looks like the absolute most vital piece of information on that page. I'm pretty sure that wasn't the intent.
I'd shrink to title down to just the title. The author, the category, "no comments", needs to go away or to the bottom of the entry.
I'd reduce the size of the summaries
I'd probably shrink the content area width...I have a hard time scanning it quickly.
I visit startupping every once and a while, but the interface for news.yc is so much better - I can scan the posts and pick out the interesting stuff quickly.
I'm on startupping's rss feed, and sometimes I might peek at an article.
But YC.News is really useful. For one thing, we're a collection of very like-minded people. But most important, the community-voting means that most of the articles that rise to the top are worth reading -- sometimes worth studying carefully!
It's usefulness that matters to me most. I hang out here because I like to be around bright, motivated people. (And no, I'm not karma-whoring.) ;-p
seems like the purpose of the sites are different, I've only been to startupping 1 or 2 times so I'm no expert. I visit YC News to get interesting news bites and the occasional comment conversation.