Portal is simply a lot of fun. Really, what more needs to be said?
Well, that it's amazingly short. The main game takes about two hours to complete. Afterwards there are additional achievements that can be completed, but they are basically sections of the main game redone to be harder.
The game concept is amazingly simple. You play a test subject in Aperture Science's Enrichment Center who is running through a test of the Aperture Science Portal Gun (or whatever it's actually called). The test - a series of puzzles involving portals - is administered by a computer named GlaDOS. During the game you get the Portal Gun, a device that allows you to open a blue and an orange portal on non-metallic flat surfaces.
The puzzles all involve the use of portals. A simple example would be placing a cube onto a switch. Instead of cliché "pushing blocks around" the cube might be placed by opening a portal in the ceiling over the button and then opening the other portal below the cube, causing it to fall onto the button.
What makes Portal special, though, is GlaDOS, the somewhat insane computer that guides you through the Aperture Science Enrichment Center's course, offering helpful advice on how to use portals, and interesting tidbits like the fact that the Portal Gun is "more valuable than the vital organs and combined incomes of everyone in <<subject hometown here>>."