Portal is a slightly dumbed down version of Narbacular Drop (ND), with lots and lots of polish. In ND, it was possible to create portals through portals, which drastically increased the complexity of certain solutions.
It also made some puzzles much simpler. Have to navigate around 20 half-walls zigzagging over a pool of lava? That was a breeze. In Portal you only get line-of-sight, meaning you had to create a pair of portals, step through, repeat. The same puzzle would have been much more challenging in Portal.