[numbsafari]

Forget about "mobile-first", this is probably the best part of Bootstrap 3:

Refactor components to be more mixin-friendly.

Honestly, Bootstrap will really benefit from being easier to fit into your own markup, rather than forcing your markup to fit Bootstrap. I mean: that's what CSS is kinda all about.

[dmix]

Agreed. This is why I abandoned Bootstrap for Zurb Foundation. I found Foundation to be much more flexible and lightweight.

Bootstrap was so painful the moment you deviate from their styles. Similar to Rails when deviating from the "rails way".

[hnriot]

I'm not new to CSS and also wonder why it was hard for you. I've found it very easy to mix Bootstrap with my own CSS.

[rubinelli]

In my experience, sometimes you "click" with a certain architecture, because it more or less maps to the way you work, and sometimes you don't. When you don't, it will be easier to adapt it if it isn't too opinionated.

[seferphier]

I am a new to css / html. Can you explain to me why it is painful to deviate from their styles?

[numbsafari]

A good example is going to be the upgrade process from 2.x to 3.x.

If you have been developing your site with Bootstrap to-date, then you've very likely been using their style classes throughout. For example, if you setup a grid on a page, perhaps you did something like this:

  <div id="home" class="row">
    <div id="content" class="span10">&nbsp;</div>
    <div id="sidebar" class="span2">&nbsp;</div>
  </div>

But with Bootstrap 3.x you'll need to change every instance of span* with col-span-*. Wouldn't it be nice if, instead of changing all of your templates, you could simply modify the LESS/CSS files using mixins?

Upgrading is just one example, another example is attempting to use Bootstrap to style an existing application where you do not have the ability (or only a limited ability) to change the HTML, but only the ability to inject a CSS "theme".

[Bockit]

If it works how I hope it does, I might have a compelling reason to use LESS instead of SASS.