Even as a self-proclaimed Valve fanboy I'm going to have to admit I'm glad that they didn't announce it. L4D2 looks nice, and I'm fairly excited about it. But honestly I don't want to hear ANYTHING about episode 3 until it is almost here. I mean like REALLY CLOSE. Haha imagine if they could have no leaks whatsoever and announced it like a day before release. But in seriousness, I just hope they don't get anyone excited over screenshots or something and get pressured to put it out when its not ready. Valve makes great games because they take ideas, and playtest them, they fix bugs, and playtest again, they make improvements, and they playtest again. After that they playtest some more! If you listen to the developer's commentaries on the Orange Box every other sentence starts with "Playtesters found that...." or "Initially the playtesters had problems, so we decided to....." Yes, it takes a LOT of time. Maybe enough to the point where it becomes silly to call it "episodic", but they are taking the time to get valuable player input from playtesting. Some of the best aspects and quirks of their games are things they wouldn't have done without playtesting. Having the pipe bombs attract zombies as opposed to just working like a grenade? Playtesting. Having challenge levels in portal? Playtesting (playtesters were finding shortcuts in levels, requiring fewer portals. Valve decided to add challenge levels to reward creative players and to add replayability). It takes forever so they can make the game to the high standards of the gaming public, for the good of the gaming public. Enough said. Good things come to those who wait.
P.S.- I do believe that Ep 3 will come out sometime. Then again, I'm naive enough to think that DNF is going to come out someday.