From this I've really found two things:
One, I think there is some massive confirmation bias at work. If a site customizes Bootstrap and it looks nice nobody credits it as a 'Bootstrap Looking' site, so as a result it is only when the defaults are used that people really pick up on it.
Second, the alternative to a site built with Bootstrap is typically not a beautiful custom designed front end, but a pretty rough looking unstyled, inconsistent mess.
Bootstrap is the new baseline for what a website should look like, it should be customized, it can be improved but there is now zero excuse for it looking worse.
1 - http://www.bootstraphero.com/the-big-badass-list-of-twitter-...
I have signed up for your beta :)
Ugh. Really. Hate the fonts (Mainly the bold condensed ones). Hate the color scheme(Yellow text on a grey-green background? Seriously? It actually looks reasonable with the colors inverted[cmd-option-control-8 on OSX]). Slow to load, at least the first time. And socialist realism posters have very bad connotations, no matter how hiply ironic you find them.
I've shown Bootstrap sites to non-developers and the response is usually that the sites look nice. Never has it been "ug. boostrap?".