What would Portal have been like if you could select what levels to play?

Chaos, most likely... and not to mention an incoherent story.

Linear progression in a puzzle game is a great way to ensure ramping difficulty, introduce new gameplay mechanics and test the user. But what if the user gets stuck on a level? You run the risk of player frustration, and breaking the flow state of the user. Not to mention if the user can't progress past said puzzle, they aren't able to experience more content.

Would Portal have worked out if it was non-linear? Most likely not, but what if it had been semi-non linear? Group puzzles together and allow the user to choose which puzzle they want to solve first, once all puzzles are solved a new group of puzzles is unlocked.

Basically, Guitar Hero formula.

Portal introduces new mechanics slowly through out the beginning levels, until it reaches a point where the user has all the available mechanics the game offers. Perhaps at this point, the linear progression could be broken up but it would still dismantle the story.

Non-linear progression in a puzzle game shouldn't be attempted unless:

* The user has all the tools available to solve puzzles. Full move set, complete equipment, etc.
* The puzzles aren't tied together by a story, or the story needs to support the player completing the puzzles in multiple orders.