Why? Because he wants to reach an audience? He didn't say "If you want 5 bucks, could you vote for it on Hacker News?". If don't like it, don't vote for it.
Well, I didn't, but I didn't think begging for votes was really accepted behavior either.
The goal around here, AFAIK, isn't to get the most votes but to have a quality discussion. This guy is just begging you to get links to his site voted up, probably so he can get more eyeballs/ad revenue.
On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.
The thing is that if it is quality and you are a HN member and your first instinct is not to run to HN and find the article and vote on it if you like it, then this is an easy way to get you to the submission and see the relevant discussion and upvote it if you like it.
There's even a wordpress plugin that detects where you come from and tells/asks you to digg/upvote items if you come from an appropriate site. (Maybe Andrew's using it?). It makes my eyes bleed, but I bet it works...