I think the most significant is that no computer game yet programmed can match the flexibility of gamers. Tabletop games which feel like they limit that inherent flexibility too much seem to be the one's most often hit with the 'computer game' tag; what is 'too much' is a subjective experience, and thus disagreements arise over whether or not a that tag is fair or not, reasonable or not.
The disagreement is much more basic than that. I disagree that flexibility is a boundless goal. Flexibility is not worthwhile beyond those bounds at which the game will reasonably run. I'll elaborate further down.
If I can't say, "Right, I'm not interested in searching this hole for the Boggle of Zoon any longer. Let's go enslave some orcs and conquer the Duchy of Fiffifth instead," then the game is going to feel too limited to me.
And here we have it. As a player, it's occasionally nice to have the illusion of freedom to do whatever you want with the game world. That illusion, of course, is shattered by the reality that you're playing a game with other people. If you were playing by yourself in your own head with nothing but your imagination, enslaving orcs and conquering duchies would be no big deal. But you're not. You're playing with a bunch of other people, and the guy in charge of the world usually has his own idea of how the game will progress.
As a DM, I am under no obligation to allow you to enslave some orcs. While I may have the capacity to let you do that (even by simply saying "Okay, you enslave some orcs."), I probably won't. Functionally, this is no different than if your computer game didn't have the ability to allow you to enslave some orcs.
As the DM, I outline expectations of my players - among which is included the understanding that you will participate in the story that has been prepared. Some DMs run things "sandbox" style, but you're still playing in
their sandbox. In this manner, flexibility - the sort of flexibility you seek - can be
damaging to the game.
You clearly don't like "artificial" boundaries in your games. I'm sure the invisible wall trope of video games infuriates you. Believe it or not, though, those same invisible walls exist in tabletop gaming. You just don't get the visceral experience of physically bumping into one. Instead, depending on the skill and patience of your DM, you will wind up being subtly steered back to the plot, or tolerated as the DM scrambles to improvise, or yelled at for deliberately jumping the rails.
Flexibility is nice. It's great to have different solutions to the problem of retrieving the Boggle of Zoon. It encourages creative thinking. There's plenty of room for this sort of creative thinking in most video games. Many encourage it. But if your idea of flexibility is having the freedom to say "My character decides to retire from adventuring life and start a vegetable stand in the local market," I doubt you're going to find your average tabletop gaming group any more accommodating than a video game.
