Arguably the most talked about piece of anything in an action-genre story is how the hero is able to overcome the odds and defeat whatever is in front of them. Sometimes it's completely overpowering a villain, sometimes it's outsmarting an opponent, but most overlooked of all is the hero managing to use their wits or own abilities to overcome some kind of an obstacle. These obstacles, quite often, will prove far more vital to the story or series as a whole than if a hero just defeats a villain using whatever they have at their disposal. Obstacles in action fiction are the most important part of the story's development, character's development, and audience's development with the series.
An obstacle in fiction, specifically an action-genre piece, can really be made up of anything, but to be specific, an obstacle is anything that a hero must overcome that does not directly influence the trajectory of the story's plot. An easy example is Aragorn having to muster the strength to ride back to Helm's Deep in The Lord of the Rings: The Two Towers (movie): the Uruk-Hai were going to attack Helm's Deep regardless of Aragorn's intervention; all Aragorn did was inform the viewer and King Theoden how impossible their odds were.
So, why is it that most of the time, when story or characters are discussed, it's often that villains or antagonists are brought up rather than some kind of an obstacle that a character has to overcome?
One reason may be that the definition of obstacle is so malleable. It can be harder to define what an obstacle is if there is no physical manifestation of what it is that the character must overcome. Aang in Avatar: the Last Airbender could not get over his inability to let go of his friends, and thus blocked off full access to the Avatar State. The obstacle here is a mental one, but, again, did not stifle the narrative's plot: the Fire Nation still had plans to take down the Earth Kingdom and wait out the year until Sozin's Comet to make their final play.
Obstacles are, most of the time, seen as a physical manifestation. It's a person, or a literal object. The most basic way of considering it is that there is a big wall between the protagonist and the thing he wants. The problem lies only with how he gets through or around the wall, not if he can get around it, because the story dictates that he must go through if he truly wants his thing. Therefore, the obstacle serves two major functions: 1) Showing what this protagonist is capable of and having them come to an understanding of that, and 2) Showing the audience what the protagonist is capable of and understanding what that means for the rest of the story.
Another reason that some may confuse obstacles with villains and antagonists is that sometimes it can be difficult to discern if a character or force is just in the way for the narrative's sake or if they're there to actively work against, or destroy, the hero.
Originally, I intended for Jiren of Dragon Ball Super to be the example I used as an obstacle for Goku and Team Universe 7 rather than an antagonist, but even then, Jiren is actively seeking to eliminate Goku. He is actively stopping the main character from getting the thing that he wants.
There was, thankfully, another example from the "Universal Survival Arc" that I could use and served a true obstacle for Goku: Kefla.
Yes, the Internet's most beloved/hated new fused Saiyan lady. Kefla classifies as an obstacle for Goku simply because her goals do not align with his, and she does not seek to actively eliminate him from the tournament, but instead to prove herself. Therefore, she is not antagonist, and she certainly doesn't have the malevolent intent to want to actively destroy him. Her innate Saiyan nature keeps her from being either of those things.
She sort of exists as another sparring partner for Goku without any real impact on his character or the audience's expectations of her or Goku's battle until Goku reactivates Ultra Instinct. From here, she is nothing but an obstacle. The video below showcases this, as we see Goku developing into his newfound form while overcoming attacks that were previously harming him, inhibiting his ability to help his team, or, specifically, achieve his goal.
While Kefla represents an obstacle in a physical form, there are some stories that do just fine with one as a mental or emotional one.
Consider the scene from The Matrix where Morpheus drops Neo into the Matrix and shows him that it's totally reasonable that people can jump across rooftops, so long as they free their minds from the artificial restraints placed upon them and enter into state of perfect syncronicity with themselves (hmm, in a way, this kind of relates back to Ultra Instinct!).
Neo fails and we continue to see that Joseph Campbell's theories on a hero's journey prove mightier than any bit of plot or narrative structure! However, herein we see the obstacle for the story: it isn't the Smiths nor is it the rules of the Matrix itself: it's Neo's mind. It's Neo's inability to see beyond what is possible.
The Smiths are the active villains and antagonists for the plot because they seek to destroy the freedom fighters as well as take it over. They work solely against the heroes to stop them and do their own thing, to better themselves in place of our heroes. They are not an obstacle; sure, Neo defeats them, but he could've beaten up some of the other Agents and it would serve the same goal for his character: realizing that he is the One and that there is nothing he cannot do within the Matrix.
It's actually not the final fight against the Smiths that show Neo ultimately overcoming this, but the last shot of the movie, when he flies away and monologues that he is going to be fighting back, which is something he'd been afraid to do until pushed to the very brink to save his friend and mentor, Morpheus. Neo overcoming his obstacle has nothing to do with physical restraints or trying to gain some new power, or even defeating a major physical threat, but rather with him coming to understand what is and is not possible. One can argue that he gets this understanding through some narrative B.S. of Trinity giving him a kiss, or one could argue that Neo hadn't been shot dead at all and he was slowly recuperating.
Either way, Neo overcomes his obstacle without any need to get a new weapon, and it really doesn't do anything to service the plot of the Machines barring down on them. They were going to release the EMP no matter what, it was just a matter of whether or not Neo and Trinity could get away before they'd be killed in the blast.
But I think the best obstacle, in any action series ever, comes from one of the most unlikely of sources: Power Rangers. Take a look at this scene and we'll observe just how this is so: