CSS rules to hide "Discover", "Trending Topics" and "Who To Follow" on new new Twitter.

twitter-user-stylesheet.css

[data-component-term="trends"] *,
[data-component-term="user_recommendations"] *,
[data-global-action="discover"] *, 
*[class*=promoted] {
    display: none !important;
}

/*
    Stop everything bouncing upon seeing modal windows under Lion when using a trackpad (non-persistent scrollbars)
    Originally from https://gist.github.com/1502938
 */
.modal-enabled, .modal-enabled .global-nav-inner {margin-right: 0;}

I just use

[aria-label="Timeline: Trending now"] {
    display: none !important;
}