Originally Ken & Roberta Williams began their Sierra Quests while trying to invent a new Genre - Interactive books.
Roberta loved the idea of having kids play a fairy tale book thus King's Quest I was created.
Her inspirations were text games which were games without any graphical trace, so on the screen you would see a description of where you are now and you could try doing things using your imagination and the text parser.
Roberta thought to upgrade these games by adding graphics and having the player move the main character over the screen (yes, without a mouse) in order to move between screens or moving in the same screen getting to various objects, while still sending commands using her text parser (move rock, climb tree [Spoilers?] etc).
Sierra quests were massive success and actually were a beginning of a genre, but as with many genre inventers (except Steve Jobs as he was perfect) she tried her hand in inventing these types of games, and many game design decisions were done using trial and error, I just want to emphasize this point as I want to talk now about the things they did wrong :-)
What went wrong with those games?
1. In King's Quest, for example, Sierra built an open world where you could go around and see all of it, but while you had your specific mission (for example: Find the Shield, Mirror and the Chest) you could wander around for hours over hours having no clue what to do next.
As a kid I wandered for days in beautiful CGA Daventry, but I can't imagine many kids these days having so much patience for it - Are you kidding me ?, After a minute or two they will ask where should they go and if you don't have an answer ready within one minute tops they will go out of the game and fire up any other shooter / flash game or whatever.
2. Dying frequently - This is not an action genre, but still there were many places where for no apparent reason one could die, a wolf, a witch or many others would appear randomly and kill you unless you fled fast enough from the screen. Not only that but there were many places where one could fall to his death easily, like stair cases, cliffs, ponds, or even just touching some items could get you killed like poisenous flowers and others. As Sierra had created quests and wanted to make them longer to play they put those death places but IMHO, to make a game longer you should put more puzzles or other goodies, not random places where one could die and will need to load the game IF HE REMEMBERED TO SAVE!
3. Dead Ends - These are one of the most horrible aspects of Sierra games - dead ends!
What do I mean ? Well, if you didn't do something (which sometimes looks optional) in an early stage of the game you will get stuck on a later stage without any way to retrace your steps and do that thing (unless you have a good save game you didn't delete already). For example, on King's Quest I you take a carrot which you need in order to lure a goat to follow you (so it will kick the Troll), but there is nothing preventing you from eating the damn carrot, and then how will you move that goat?!
Another example: In Space Quest you need to take a crystal shard at the beginning so later on you will be able to use it in order to finish a puzzle, but if you fail to find it (which is easy) you might still take that one way elevator and get stuck on the other puzzle forever!
4. Treasure Hunting - Some of the items you need to finish your quest aren't given to you by any character in the game but randomly appear on random screens along the game, so as the game goes you just wander around trying to find an object which might appear on a random scren - frustrating as you don't even know what you need you just try to take anything which isn't nailed to the wall..
5. Pixel hunting - Some of the items you need to take along the game are so small , so they literraly are a single pixel big (in those days when the game resolution was so low), and it is frustrating to death to just miss one of those (the whistle on kq4 ?) because of that factor.
6. Damn hard puzzles - Most of the puzzles are good and reasonable, but there are some puzzles along the game which are totally unacceptable, like spelling Rumplestitskin's name in cypher mode !?
7. Timed scenes - I personally hate this one, when a timer appears on the screen and you must accomplish a task in a timely manner, It happenned alot on King's Quest III, where you needed to accomplish tasks between Memnnon's appearances, walking that cliff fast without falling then searching for ingredients and coming back in time was just frustrating for me, not challenging.
8. Text Parser - The early games had no option for a mouse so I understand that a text parser is a must, but still more work should have gone into it (And I know that it is complicated) In the text-parser quests I had to just try any combination of things I could think about, for example, I see a rock so what should I do with it? "Pick rock", "Push rock", "kick rock", "Shove Rock" you get the hang of it, I remember myself as a kid trying so many things on so many objects which was sooooo frustrating.
Lucasarts on the other hand took the idea of quests from Sierra, but they have built their engine and had an upgraded philosophy which was meant to fix these flaws, well, not on the spot but they fixed their gaming philosophy quite fast.
Although I said from the beginning that Sierra is not to be blamed for the above (IMHO) flaws as they were the first to invent this genre which means that they couldn't get those games perfect on the first releases, I still think that they could have learned better and changed their game's philosophy much fastre than they have.