I have two follow up thoughts after thinking about this some more.

• It is probably going to be necessary to provide ways of segregating content at the blockchain level. For example, Steemit.com is probably not going to want to render all of the posts from a Twitter like service. The Twitter like service is not going to want a bunch of streaming videos to show up, etc.
• Whale voting power is going to be a 'hurdle' for a third party site to truly integrate with a Steem blockchain voting + reward system. If a site uses the blockchain for comments + votes, and even if they get tons of people to signup and use the system - there won't be any 'rewards' unless they can attract existing whales to their site or convince their user base to purchase massive amounts of SP. To solve this, I would recommend that the comment/voting system still be functionally attractive for use - even given a user base filled with all low SP users. Basically in that scenario - none of the comments actually 'pay out', but hopefully the comment/voting system still provides enough functional value to be 'worth its use' even without the rewards.