One reason I can forgive Valve for not finishing Half-Life 2

That reason would be Portal.  Half-Life/Half-Life 2 are great games, but Portal is a literal game changer.  I never could have imagined a puzzle game that is so mind-bending and unique.  One of the best feelings I've had in gaming has been the satisfaction of solving puzzles--that initial frustration of not being able to figure it out, the trial and error, and then the eventual victory of solving it.

The first time I played Portal I was a little confused.  You get a gun and there's nothing to shoot?  That's weird.  Not shooting stuff (other than walls) is what made Portal so fun and different.  You really have to think outside the box to figure out how to advance.  You have to visualize where your Portals will lead you and sometimes time them just right or multi-task while flying through the air.  The goofy sense of humor in the games makes the frustration enjoyable.  The fact that there is a computer talking crap to you while you flounder around makes for a great game.

Portal 2 took an awesome formula and made it even greater.  The introduction of gels that have different effects (speed up, bounce, or create a surface to make portals on) made for an expanded array of options and challenges.  The co-op mode in Portal 2 made puzzles twice as puzzling as now you had to think of 4 portals rather than two.  There are still plenty of laughable moments in Portal 2 as you work your way through the history of the Aperture science laboratories.

Sure, I still want to see a finale to the Half-Life series, but I'm happy that Valve took the time to create Portal.