If you want it, Bootstrap probably has it.
Before you write your own CSS, check and see if Bootstrap already has it. Examples I used in this presentation (there are many more):
class="pull-right"
or class="pull-left"
Add JavaScript without writing any JavaScript.
You can create interactive effects just by adding the right class names, id names, and/or data attributes to components. Examples from this presentation (there are many more):
See the Yay Bootstrap! source for comments on troubleshooting Bootstrap JS.
Be different, just like everybody else.
less - it's magic. It lets you use variables and mixins in your CSS. Want to define a @green instead of remembering the hex code everywhere? Want a lot of your elements to have the same .rounded_corners_and_orange_background()? You can and it's the best. Bootstrap is built on less -- this is what makes customization such a snap.