King's Quest, Space Quest, Police Quest and Quest for Glory are perhaps the most iconic, but there were so many more that had their charm; the historic pseudo-treatise Gold Rush!, the Disney adaptation Black Cauldron, the in media res Manhunter, the young gamer focused Mixed Up Mother Goose. Even I, a huge Sierra fan, haven't played them all. That said I believe Sierra's success is really based on four key principles of design.
You Die, They Don't: The number of ways you can die in any given Sierra adventure game is impressive. In fact I would argue that this was a feature of these games since a number of these events were entertaining in their own right. Conversely, your character rarely killed anyone else. Even the Quest for Glory series had a fairly low body count compared to modern action games with most of the fights being avoidable either by escape or alternative puzzle solution.
Fairy Tale Logic: A common criticism of old adventure games was the annoying tendency to have puzzles that were not solved through scientific methods. I agree it was frustrating to find out that not doing something earlier in the game could make a later obstacle unsolvable. Thus forcing the purchase of a hint guide (the original micro-transaction!). However, someone well versed in Greek Classics, Arabian Nights or other myths and legends would probably be able to figure out a lot more than a science geek. So, depending on your scholastic background YMMV.
Changing Mood, Same Theme: This one is a bit intangible, but I'll try my best to clarify. I'm not referring to the jarring transitions between games - Space Quest V to VI or King's Quest VII to VIII. Rather what I'm getting at is the gamut of moods a single game explored. To give you an example the Green Isles in King's Quest VI each have their own theme. The Isle of Wonder feels very whimsical compared to the mysterious Isle of Mist, or foreboding Isle of the Beast. Then there is the terrifying Land of the Dead visit juxtaposed against the Disney romance that makes up the central theme. It's a hard point to nail down, but in some ways I think this particular aspect of Sierra games is what made them so memorable.
Now, you might be wondering why these aspects of game design matter. Well, the truth is they're pretty much ignored. In fact I'd go so far as to say the opposite of all of them are the dominate design philosophy at the moment. Things go in cycles though and the pendulum inevitably swings back. AGD Interactive has had some success with their free high quality remakes of Sierra titles and the aptly named Phoenix Online is making an all new King's Quest so there must be a demand for Sierra style games. While I doubt Roberta Williams will play part in the future of video games her influence will remain as long as the games she made are remembered. The king is dead! Long live the king!!