Do you think that subscription based sites, that offer software as a service model, should be marketed differently than a content specific site. Beyond contacting bloggers, blogging yourself, and going to trade shows and events, how would you directly engage a paying client base to "spread the word" beyond affiliate marketing stuff?
If you have some people already using your software, get them to do a testimonial for you. how did it help them? what was there problem? how was your product better than the others?
Its just a small tip but It works especially well if you have some well known clients you can stick the logo of on your site.
I don't think it would be that much different really. I would still have a on-topic blog and use that to help engage others in the domain - so it's still a content site in that respect.
To get customers to spread the word you'd have to follow Seth Godin's advice: do something remarkable. You don't have to do something unique but you do have to do something that makes people take notice and want to tell their friends about it.