There were also compatibility issues with the "CSS box model", where IE6 didn't follow the spec at all and broke nearly every site in every other browser because elements ended up with different sizes.
They fixed that with IE7, and we finally entered the utopia CSS promised with every browser agreeing on how to size elements.
And then everybody realised the CSS defaults were wrong, and the IE6 behaviour actually made more sense, and now pretty much any complicated site will opt back into the IE6 box model.