Honestly, I really like portal. Not because it was too short or that there was a massive amount of graphics. It was simply the game play aspect that made it appealing. Like any "new" technology, Portal was really nothing more than a tech demo on what the capabilities are of the source engine. Consider it a playable advertisement to game makers.
I like that they offer new ways to play a game. I think if they release another portal, it will be more focused on all the things the first one wasn't. It's already got the attention of the gaming community, next is to wow them into believing it's the only way to go.