Having all of the content available in one play through = a Linear Game without consequences of player action.
Actually...no, that's not true at all. It is perfectly possible to create meaningful consequences without "walling off" sections of content. Difficulty level, for example: if you join up with Faction A, then Quest X should become much more difficult to accomplish because the group or faction providing you with Quest X disapprove of your association with Faction A. They could force you to "prove" your loyalty by assassinating an important (but nonessential) member of Faction A, and until you do that you cannot proceed through the quest. At that point, you can attempt the assassination, but if you fail or are discovered in the act,
then you lose access to Faction A.
That's just one example, but what I'm essentially saying is: players should always be given choices, be given chances - that way, when they do make a mistake and lose access to some block of content, it has meaning, permanancy. There should never be any motivation for you to regress into a "quicksave" mentality, where you're loading up your last save to "fix" a choice you made. There should always be enough of a "way out" to motivate the player to stick by their choices and explore the outcome. Otherwise, it becomes a metagame of "save and see which choice has a better outcome, then load up the save and pick that option". That destroys immersion more than this percieved "linearity" ever could.
Just to be clear: I am in no way saying actions shouldn't have consequences. I am also absolutely against making it easy to access every quest in one playthrough. All I'm saying is, with enough skill, it should be
possible to access all of the content in the game in one playthrough. Otherwise, you're not actually risking anything by making decisions - you're riding a predetermined flow chart.