How? Give a reason, don't just make some unqualified statement. What could a PC exclusive Portal have that a multiplatform Portal couldn't? What did the original Portal lack due to the fact it was also available on the 360?
No one can say what portal lacked except the developers and they won't do that. But the big thing we can offer conjecture on are mice and keyboards. The level design was probably affected by the lack of mobility the console controls would offer the players in game avatar.
Also right now at least, there are tons of things the consols lack the PCs have. High resolutions are one of them. Most consoles games will be 720p because its what most TVs are. This affects the players field of view. Namely, draw distance will be lowered because even if they rendered further away, you would not be able to see what some one running the same game at a resolution of 1080p would see. A good example of this was the preliminary plan for Bad Company 2. DICE developers said straight up that until the PC Beta testers said they did not need prone, the PC version was getting prone and the console version of the game would only get crouch. This is a big deal if you like to snipe, or even if you're a douchebag. I mean a camper. They said that when Prone was enabled on the 360 version of their game, you could not see people laying down until you were dead. On the PC, you would see these guys if your resolution was high enough. The camping problem could be mitigated. It is just one minor feature, but its a good example on how the way a game plays can be affected the machine youre using.
sorry about the wall of text.