How do you get people to care?

You can't force people to like your site, be interested in your content or return. There's no magic bullet (if there were, I'd be able to pay off my student loans.)

In your particular case, I would politely suggest that there's nothing particularly unique about tagxt in terms of user-experience, nothing novel in your approach to the format. As such, and as with the many forum/bbs sites already in existence, your primary draw will probably be the community and the kind of threads you have... I'm hanging out on Hacker News because the programming/hacking content interests me, for instance, not because the layout of HN is particularly clever. If this was Sports News, I wouldn't be quite as interested. It's all well and good to sell the sizzle but at some point there also needs to be a steak.

tl;dr give them something to care about or wait for the community to build itself.

How about not spamming HN?