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.
I'd be sure to offer free trials/demos so people can see the value first hand. Create videos of how to solve common pain points quickly by using your software.