Portal 2 isn't merely a sequel, it's a viable application of what was essentially a trial run. Valve Software has stated that it never expected Portal to gain the extended following it has, which is why The Orange Box acted as a safety net. A most unnecessary safety net, since Portal managed to eclipse both Half-Life 2: Episode 2 and Team Fortress 2 in popularity. And now Portal 2 has evolved into a standalone title that does just that: standing alone.