In order to say something about HL3 again:
Maybe Valve has also become careful when observing how their competitors were doing in the meantime. Just consider Bioware which used to be a guarantee for top-notch RPGs until Dragon Age 2, Mass Effect 3 or SWTOR got released. Or as another example, id with Rage (actually not a bad game, but fans simply expected more). Players are possibly spoiled after many high quality releases, and as soon as you deliver something more average, it's already considered being "bad" and flops (which is a relative term since the media already speak of a flop when a highly anticipated blockbuster title sells "only" a few million copies).
It has become more difficult to satisfy player demands, and as long as HL3 can't live up to those expectations, I doubt they will dare to ever even consider a release. However, they are about to suffer from the DNF syndrome: The longer they wait, the higher the expectations are - and the more likely it is to disappoint them.
I have read about many speculations regarding HL3 recently, e.g. that it would actually depart from the FPS genre and tend towards an adventure, quest/mission-based and with reduced action. Whether it's true or not I can't tell, but I don't think it's such a good idea to keep fans waiting for so long. I prefer "transparent" development, i.e. devs involving the community, telling them where things are heading, asking for feedback and put it to good use to apply changes before release. From personal job experience, I can say it's NOT a success-promising model to create a game which is perfect in your own opinion, but without asking those you are making it for. In the end, you want
them to buy and play it, and in order to accomplish that, they need to
like it. So show them something! By the time you just throw it at them and it fails, it's too late. Avoiding damage is always a lot better than trying to repair it.
However, I guess since Valve has never done it like that, at least not regarding Half-Life and not as far as I can remember, I doubt it will be any different for HL3 in the end. Personally, I don't expect to see it before 2014 or even later. We can safely assume that development is going on for a long time already and should be in an advanced state, provided they haven't thrown it all into the garbage and restarted from scratch (which isn't too unusual by now, unfortunately). Let's hope the Half-Life tradition continues and we'll get a game that deserves its name. It doesn't need to be a milestone again, just a great game.