The UI for adding elements to the page is not intuitive. Some little "add here" thingie shows up and when I click then it gets added. Took me a while to figure out, I expect drag and drop.
It's cool that one can build a blog/HN, to-do list, but from looking at the builder I don't know how. But the bigger problem is the concept that I should build HN or a to do list from some low-level pieces. I think for designers it looks reminiscent of dreamweaver, where you couldn't build all that much without coding. For others it's probably too many buttons.
I think you should start with a finished template, let's say of HN and edit that to make a blog where users vote on one writer's articles, or a to do list, where you just remove the up/down voting and leave the submission form. Or to make some enhanced HN, add up/down voting conditions when a user just clicks the link, add "label" votes where you can vote for discussion article creates, not just article, then add site sort by quality of discussions.
An app builder is only as flexible as it's presets and templates, but starting with a blank canvas gives the impression that it's some professional tool with a big learning curve. I think you should underline that it's not flexible but enables you to prototype variations on established paradigms easily, so display finished template first.