Things that I like about bootstrap are : stability, performance, easy custom builds, community support (almost every problem has been addressed on Stackoverflow), ecosystem (themes, extensions, tools). The design out of the box might look cookie cutter, but if you know basic CSS, it is very easy to customize it with minimal effort. Just lookup examples on codepen/stackoverflow.
Some people complain about bloat but this is amazing : https://getbootstrap.com/docs/3.3/customize/ . I have public facing websites with thousands of visitors per day. The performance rating is better than 90% of the websites.
Truly want to thank @mdo @fat @cvrebert and the rest of the team for this amazing project.