"At the dawn of the universe, there was nothing. Then... boom. The big bang sent six elemental crystals hurtling across the virgin universe. These Infinity Stones each control an essential aspect of existence."
The Infinity Stones and Thanos's pursuit of them were a well-executed concept that drove the underlying story behind the first age of the MCU, now known as The Infinity Saga. There are six Infinity Stones: The Space Stone (a.k.a The Tesseract), The Soul Stone, The Mind Stone, The Time Stone, The Power Stone, and The Reality Stone (a.k.a The Aether). One Stone alone should be more than enough power to make you unstoppable, yet heroes and villains throughout the saga consistently overlook the immense power at their disposal.
Ronan
The Guardians of the Galaxy may have helped, but Ronan The Accuser was ultimately defeated easily because the film had such a light-hearted tone. The Collector explains and visually demonstrates how each Stone "can be used to mow down entire civilizations like wheat in a field," yet when Ronin wields an Infinity Stone, he still loses.
Dance off aside, he only uses the incredible power at his fingertips to harmlessly shove people to the ground. The movie was a lot of fun, but Ronan barely registered as a villain.
Vision
It was sad to see Jarvis go, but it would be selfish to hold him back from pursuing his destiny. He became Vision, and Paul Bettany got to show his face kind-of. He quickly proved to be one of the most powerful Avengers, if not THE most powerful, thanks to being the physical embodiment of the Mind Stone. But he, along with Scarlett Witch, whom together give new meaning to the phrase 'power couple', rarely help out in battle, or give us much of a demonstration as to what they're truly capable of.
He hasn't really done anything cool since he lifted Thor's hammer in Avengers: Age of Ultron and is frequently pushed to the sidelines, probably because he's too powerful for the Avengers to be in any real danger while he's around.
Thanos v The Avengers On Titan
Thanos makes good use of the Infinity Stones for his purposes when he confronts the Avengers in Wakanda at the end of Avengers: Infinity War. The fight two Stones earlier on Titan was thoroughly entertaining, but Thanos didn't seem to realize how much power he was already wielding. You don't need to control time and thought to win a physical fight if you can already control space, reality, and life itself.
He could have saved himself a Soul Stone trip and still been untouchable. And yet the fight plays out like the one in Captain America: Civil War: a lot of awesome but ultimately meaningless noise. At one point Thanos aims the Infinity Gauntlet directly at Iron Man and blasts him with a beam of energy that his shield can somehow withstand.
Thanos's Plan
There is certainly some logic to Thanos's psychopathic plan, but considering the godlike powers he possesses after uniting the Infinity Stones, his plan to kill 50% of all life in the universe shows a lack of imagination and understanding, as well as not being very nice. He claims he's doing it to create a better universe, that isn't overcrowded and at war, and after the Avengers come at him again in Avengers: Endgame, he decides to wipe out all life in the universe and create a new one teeming with life.
If his actions were all for the noble pursuit of peace throughout the universe there are infinite ways to achieve that without shedding blood. He could make every planet in the universe a habitable paradise and put them all right next to each other so they spell THANOS RULEZ, or just make travel throughout the universe instantaneous. You can pretty much apply all the 'Why Did/Didn't He?' questions people ask about God to Thanos.
Nick Fury And The Tesseract
It's one thing to use an infinite power source to make handheld weapons instead of fuelling the entire world's economy, so nobody has any need to fight. It's another thing entirely to see that these weapons manufactured by SHIELD and/or HYDRA never kill anyone of note or even shoot straight. Again it comes to people firing beams of light at each other that do whatever the writers tell them to do, as with the battles in Harry Potter.
It's also Nick Fury who decides to use the tesseract to try and make a stargate, thereby alerting the universe to Earth's existence and the fact they're messing with an Infinity Stone. It's disappointing to see no evolution to the world's technology to parallel what's going on with SHIELD. Stark Industries too. Stark and Fury should have immediately jumped on the tesseract as an opportunity to scrap the arc reactor and find a new sustainable way to power the world's economy.
Hulk/Iron Man Snap
The good guys proved to have a similar lack of imagination to Thanos when it came to playing/being God. But in the case of the righteous defenders of the Earth, they should have even more reason than Thanos to make big changes to the universe for the better.
Maybe Thanos wouldn't have shown up and blown up their lovely Avengers building if Hulk had put more thought into his snap and created his own kind of utopia while bringing everyone back.
Don't Touch!
In Johann Schmidt's defense, there was no known science to explain what might happen if you grab hold of a Stone of infinite power that controls an essential aspect of the universe, much like how there's no way of knowing what might happen if you went through a black hole. But as with entering a black hole, common sense dictates it's probably a bad idea.
It's one thing if you're a protagonist of a powerful ancient race, and your best friends in the whole world are there to support you, but if you're just a regular human, played by Hugo Weaving and aptly nicknamed Red Skull, clearly nothing good can come of it. He should have just made some decent weapons when he had the chance.
The Time Stone
Doctor Strange takes full advantage of the Time Stone twice in Doctor Strange. He rewinds time at one point to do-over a fight and very memorably forces Dormamu into submission in the film's climax. The Ancient One warned against frivolous use of the Time Stone, as many characters are forced to do in movie franchises involving such devices, but Doctor Strange's actions didn't seem to have any negative side-effects.
Endgame demonstrated that time travel within the MCU is fundamentally incapable of rewriting the past, so it seems unrealistic the heroes don't turn to Doctor Strange to stop time every time there's a problem.
Odin and the Tesseract
After the Avengers achieve their first victory in The Avengers, Thor brings the all-powerful tesseract back to Asgard, where Odin uses it to repair the Bifrost before sticking it in a vault. It seems like a waste, but it keeps it safe. But when Malekith shows up to end the universe using the Aether in Thor: The Dark World, Odin, and Thor seem to forget it exists.
When Malekith descends on Asgard the Aether is living within Jane Foster's body, so it makes sense that they don't weaponize it. But there's a perfectly good Power Stone sitting in a vault that could effortlessly keep Malekith at bay. Frigga loses her life needlessly to this oversight. And locking it in a vault doesn't prevent it from getting stolen by Loki in the next movie.
Loki
In The Avengers, Loki serves as Thanos's servant to go and retrieve the tesseract. But in 2012, Thanos had no Infinity Stones. Once Loki had it in his possession, he would effectively be more powerful than Thanos, yet he goes along with Thanos's plan to try and take over Earth and defeat the Avengers, which doesn't work.
In the end, the real problem with the Infinity Stones is that they're so powerful anyone who has one should be impossible to defeat. It would make for some really short movies though.