I really can't see how you can release extra stuff later for something like Half Life, with L4D and TF2 it's a pretty simple matter, because of the way they're set up.
TF2 can have stuff added because it's dynamic, and there's not much story, for example, if your team wins in one map, it doesn't change how you're going to be doing in the next map. L4D can have extra stuff chucked in because it's episodic, and there's stuff happening between episodes you can explore with DLC.
But, with Half Life, the idea is a single experience that can be played from start to finish without being chucked back to a menu screen. If you cut it down into several episodes like L4D, then you're basically back to episodic releasing! That's the only real way to add new content that isn't just bug fixes. It's a loose-loose situation! D=