The first argument has been brought up a lot of times, but it doesn't seem to be true, ever. Every time someone made something unique, we had few new players try it out and the rest is forumers or Blockland regulars (or both, for that matter). As for the second, youtubers have already shot down Blockland with videos such as "BANNED FROM BLOCKLAND?!?!" and "BLOCKLAND SUCKS/IS DEAD", so don't bother. Unless you're a popular youtuber.
We would have to be consistent though, it may take awhile. Also, we would have to saturate YouTube with positive Blockland content. Over time, if people see videos of players enjoying various servers and add-ons, they would possibly give it a second thought and retry it. Just my opinion, but it's worth a shot.
I think a proper "let's build" would be good. Though, it would have to be done correctly. It could be incredibly boring if it's not done correctly. As for let's plays, the ones that have been done just didn't seem anything special. At least not at this time. The ones that do exist (that I've seen) were mostly just jail rp's or one of the generic game modes. None of them were really focused around building.
You know what would be great, if someone documented the construction of a really large build. That would be something I'd like to see. Like someone making a large city, and each day they document what was new that day and after a time there's a giant city that has been made. I'd personally really like to participate in something like that. It would be a really good opportunity as a community project as well.
Yeah, I'm not sure exactly how to make a let's build exciting or interesting. I have designed some buildings in real life and would like to do a series remaking them in Blockland, perhaps doing a timelapse of the actual building portion of the video. Also, I've been wanting to do a survival let's play series in Blockland using RPGx, but I'm not sure if that would work.