Thanks for the followup!Ok, here's some reasons
1. Portal: Fantastic problem solving based gameplay, fluid motion on my personal favourite engine, and above all, a fantastic narrative from Glados, the crazed computer. Despite it's linear nature, I've played through it 5+ times, and it never gets old.
I got Portal when it was free on steam (several months ago) and I have to agree it's an incredibly fun game, and the use of the portals is one of my favorite mechanics ever. It is awfully short though and I wanted more from it, though the challenges were quite nice.