* They came with a mail and chat (IRC) clients, a download manager, a set of browser dev tools, and in the age of limited internet traffic all of that was smaller than a single download of Firefox.
* Their dev tools were the first that allowed remote debugging. You could run Opera on your phone (Symbian, Windows Mobile, early Android) and debug your website from a computer.
* They were the first browser to sync your bookmarks, settings, history, extensions across devices.
* They were the first to add process isolation, albeit initially on Linux only. If an extension crashed your page it didn't take the whole browser down with it. This was later added first by Microsoft in IE8 and then by Google in Chrome.
Their browser was a brilliant piece of tech and a brilliant product. Too bad that the product couldn't survive under pressure.