Indeed, back then. Now however their games are, quite frankly, hollow. Both in writing and in gameplay tbh.
Examples:
Left 4 Dead 1/2: run and gun, spray and pray on everything etc.
Portal: Cool gimmick for the time but that's all it has going for it. The story isn't that great and by that I mean there really is no story.
Half-Life series: Story so complicated there might as well not even be one, let alone the fact that, as Donner said, it is quite linear.
Team Fortress 2: Started out pretty fun, has now devolved into a grindfest for better weapons and stupid hats, Azurik was more fun and felt more rewarding.
I will agree that Left 4 Dead is one of their lesser games.
I think that the story in Portal is still waiting to be uncovered. You start off not really knowing what is going on, but later on through the game you get more hints, but a lot of things are still unknown. I think that Portal 2 will shed some light on things.
Half-Life has an amazing story in my opinion, and that is what kept me playing it.It probably had the best story out of any fps I have ever played, granted it is linear, but most fps games are.
I agree that Valve has lost some of their luster with games like Left 4 Dead and Team Fortress 2, but I think they will be revitalized in the years to come.