Normally when I share links to friends off the chain I'll drop them a link of Busy, not because of anything but it's probably the most appealing frontend visual-wise...I hope I'm not too far off but Steemit's UI feels like Reddit's old UI, minus the responsiveness, plus ads that steal the attention on the top. And you reminded me that there's no sign-up invitation on Busy! Sure, there's one on the right side (taking the place of the recommended blogs section if you're logged in), but you don't get a reminder that you have to sign up to comment or vote...

Guest comments can possibly be implemented on the frontend-side with Disqus or similar services, but it will be a problem when it comes to moderation or abuse because remember that most Steem frontends don't have real staff! Theoretically speaking the blockchain can also add support for guest comments, but it opens up another door for abuse and breakage.

Personally I think the problem of design here is how the thing should look like. Colours won't help here, because, honestly, possibly, people aren't looking at it because it looks like a part of what people have been adding to post signatures, just less visible. The 100% visible solution is to make the comment box more visually appealing (at least take up the post's width, please), visible 100% of the time, and grey out the whole thing telling them that they need to sign up to comment. When you can't see the comment box, do you even consider the chance to comment? Highly no...

At this rate of rant I should fire up my own blog site as a Steem frontend and just share that lmao.