OS widgets provide a user familiar with those elements hints and clues about how to use a program. A menu is a menu is a menu no matter the application. Bootstrap provides a very similar framework.
The problem is that we don't interact with public facing websites in the same way we interact with websites in the same way as desktop applications. The level of complexity tends to be very different and design is far more important for a website.
I can understand using Bootstrap for an admin interface; it provides that same recognizable framework for a user. For a public facing website, however, I think using Bootstrap is almost abhorrent. Whenever I see that very distinct black navigation bar across the top I have an urge to leave a website.
I understand that you can of course modify Bootstrap but to be honest I think the design choices are simply wrong for most sites. Bootstrap is great for web applications but for a blog... seriously?
Anyway, enough ranting. I actually really like Bootstrap and I'm using it currently but I know I would not use it for my own personal blog or public facing site.