Many people have commented saying that their primary concern with Bootstrap (or what holds them back from liking it) is the default styles. There's a lot being done to combat the wave of bland Bootstrap deployments and not everyone chooses to go vanilla.
You can try:
- Bootswatch, free color schemes: http://bootswatch.com/
- Lavish, a way to apply colors extracted from images: http://lavishbootstrap.com/
- WrapBootstrap, a marketplace for premium Bootstrap themes: http://wrapbootstrap.com/ [shameless plug]
Check out Built With Bootstrap for great examples of sites that are thinking outside the default styles: http://builtwithbootstrap.com/