Why are people comparing this to the Portal update that announced Portal 2, or complaining that it was too easy compared to that one? There are a few key differences.
1. When Portal was updated, something was obviously up. VALV[sup]E[/sup] had no clear reason to update the game, and the update was not a gameplay update or bugfix. This code, on the other hand, was in the game FROM DAY ONE. There was no update or anything to grab people's attention.
2. Portal's update was extremely bizarre. Radios everywhere? People would've been fools not to check it out. This code was disguised as typical official-looking bullshit. People didn't look into it, because it's exactly the sort of important-looking letters and numbers that developers like to stick in to add to the aesthetic of the game.
This code had to be simple, because they didn't give people a convincing reason to look for it, or even evidence that it was a code at all. If it'd been any more complex, people might've simply disregarded it as not a code at all, but simply random letters and numbers to look cool. The key challenge was not supposed to be in decoding it, but in identifying it as a code at all. And the fact that it took this long is pretty strong evidence that it worked.
This wasn't too easy. It was exactly as difficult as it needed to be.
P.S. Thanks