At first I thought the same. I thought that the complex nature of the puzzles, yet easy to learn mechanics, along with the brilliant writing just made it perfect, had no need for improvement, besides maybe a few more levels.
When I saw the E3 Trailers for gameplay, though. I realized just how much more creative and complex and fun to play the portal puzzles could be. That and I had faith in how the story could take place, yet not suddenly become a full narrative. That's never Valve's case, anyway.
After all, when I first saw Star Wars as a kid, I was enamored and thought that even though I was excited for more, I didn't think it could be topped; but I remember watching EmpireSB with my Dad... and yea. Things thought of as masterpieces can be outdone- definitely