Valve just gets too ambitious to keep a short schedule. Major engine updates (EP1 featured one, EP2 features one), all new animations for the "cutscene" actors, too many new assets needing to be created. Its obvious they didn't really plan for this or they would have made some more useful, generic actor animations that could be used over and over for the cutscene bits, and other such things. Keep the need for new customized assets to an absolute minimum. Record dialog, build maps, script them up, and shove them out the door.
I still think HL2 is a little too "high brow" for episodic. It requires too many levels to make anything meaningful, so instead of making a map or two and calling it an episode they need to make 10-15.
I think a more action-oriented FPS would do better in episodic format. Ok, go with me here: You make a zombie apocalypse episodic FPS. Your main content push is up front in the first episode. You make a bunch of dirty urban textures, blood splatters, sounds, a few zombie models, the main guy model, maybe a couple non-zombie people to talk to/rescue or whatever. So now you can develop an "overall story". This is your story arc that connects all the episodes together, like a TV show season. Each episode isn't just a slice of the overall story. It will contain parts of the overall, but also have its own sub-plot. So for example in Episode 1 your girlfriend falls ill and needs medicine, of which your little survivor colony has little. You need to go out, fight zombies to the hospital and find some meds to bring back. Thats the sub-plot. While you're there you overhear a weak radio broadcast about another colony somewhere. Well, a few episodes from now you'll be trying to move your colony to the new one, thats the overall story. So in between you need to gather supplies like vehicles and stuff, then a big "season finale" where you need to get as many vehicles full of people to this new place.
This is just an example, but for episodic it would work well. The "home base" map can be used over and over again, its all in the same city so texture and models can be used multiple times, zombies are used over and over (and each episode could add 1 more to the mix to keep things fresh), no need for new weapons for the most part. And after the finale you'll have a new location so season two can have a bunch of new assets created for it.
HL2 just isn't set up properly to make efficient use of an episodic format.