Just like there’s a balance between too much action and too much information in fiction (a big cornerstone point that I thought a lot about for my book), a balance between external conflict and internal conflict, and a balance between characterization and plot, there should also be a balance between high-stakes obstacles and easy hurdles.
The best case scenario in any plot is an obstacle that seems just impossible enough, and then is acted upon in a surprising way, bringing about delight and relief in the reader. The two extremes on the scale of obstacles: the wimpy obstacle that is overcome too easily, and the impossible obstacle that kills the reader’s sense of hope.
The first one is bad for an obvious reason: you always want to be playing up your stakes and tension, especially as you move toward the climax of your story. If a bad guy goes down on the first punch or the secret journal that simply can’t be found is…in the attic, well, that’s a bit lame. You don’t lose your reader if you have one or two of these easy obstacles, but if the reader gets the message that no challenge is really all that challenging in your book, you will lose them after a while.
The latter problem is, actually, what I tend to see more: the obstacle that is so impossible, so implausible, so high as a hurdle, that I give up almost before the character tries because it strains my suspension of disbelief. While I applaud writers for making big, high-stakes obstacles and putting them in the paths of their characters, the protagonist must always stand at least a fraction of a snowball’s chance in hell of achieving the objective, or the reader will click off. There are those “impossible dreams” that are darn difficult to achieve, and so the journey of that process is worth sticking around for, and then there are those goals that are simply impossible. Aim for the former. (An off-shoot of this impossible obstacle is the protagonist requiring something of a character, and that character just saying flat-out: “No.” That does not give you much room to strive toward the goal, either, and, in most cases, strikes me as extremely arbitrary.)
To strike this balance: Set the bar high, but give your character a fighting chance.