Pro Merges first-person shooting mechanics with platforming and puzzle-solving
Portal uses an engine that was meant for a first person shooter and creates puzzles that need solved through various stage with the use of platforming. While not the very first game to merge platforming with and FPS the puzzle aspects make it unique in that no other game had done this specific combination yet.
Even though the only organic sentient being in this game is the player's character, this game features some of the most memorable and intriguing characters in fiction or in gameplay. GladOS obviously, but also the drone robots that sweet-talk the player to a sudden death (and then forgive you when you dismantle them), the clues left behind by former test subjects, the abandoned hints of what the lab used to be like. It tells a story without telling a story, and immerses you more than most other games can ever hope to achieve.
Con Portal mechanics not used much to their best ability
Certain parts of the game sees the player using portals to manipulate sentry guns that fire rockets in order to progress in that stage, Mechanics like this make for a really enjoyable way to manipulate ones environment but sadly are not used too often.