Broken Base: The season is a fan favorite. The theme song, on the other hand is very polarizing, especially considering that there was a contest to see which song would be used, and this song was the more popular choice and the song used wasn't even one of the choices. (Although the actual theme song did arguably better fit the theme of the series).
Complete Monster: Venjix is a sentient computer virus who was created by Dr. K and became rogue after being unleashed on the world's computers. After being unleashed he would create an army of robots and use them to exterminate 99% of the human race. When the last elements of humanity hole up in the domed city of Corinth, Venjix makes repeated attempts to destroy the city in order to wipe out what is left of humankind. To achieve this end, he enslaves people to work in his factories to make his machines and forcefully roboticizes them, even brainwashing Dillon's sister into becoming Tenaya 7. Venjix is also a user and abuser of his minions, to the point that he will kill perfectly loyal minions for no other reason then he can and discards any henchmen he no longer needs. After Tenya goes against him, he has her forcefully turned into his loyal slave Tenaya 15. In the finale, it's revealed he's taken over the minds of half the city in order to force them into compliance with his orders. Possessed of no good qualities, and with one of the highest body counts in the history of the franchise, Venjix is among the worst that Power Rangers has to offer.
Dr. K keeps a laser cannon in the fridge - not stored, but primed and ready to fire. She also made an experimental weapon which turned out to blast clothing off, and can control the base's ventilation systems with her violin. And yes, all of these get used against bad guys multiple times. She also once simply plugged her violin into an amplifier to create an impromptu sonic cannon at one point, and is so lacking in social skills that most of her statements come out as so bizarre they're awesome.
Gem and Gemma, emphasis on CRAZY, but still awesome in their own way.
Evil Is Sexy: Tenaya 7, who dresses like an action figure and looks like a model. It's kind of hard to hate an antagonist that looks like Tenaya 7.
The first time the Megazord is formed, Scott says "Vectorscope, online!" A vectorscope is a monitor for television signals that measures the color saturation. Since the 'eyes' on the Zords are stated to be high-powered cameras, the vectorscope lets them monitor how much color the camera signal has.
Gem is played by Mike Ginn. Which was the catchphrase of the silver Tiger zord back in Go-Onger. Hmm.
The space-shuttle dagger Gold and Silver use was called Rocket Dagger in Go-Onger, but renamed in-universe to Rocket Booster in light of a recent fatal stabbing incident in Japan. RPM takes it in a different direction by naming it the Cloud Hatchet.
One of the demo themes contained a certain line that, at the time was quite sad, considering the show was facing cancellation. But now that the show's been renewed, the line "Don't they know we'll find a way / Power Rangers are here to stay", seems almost prophetic.
Mike Ginn starred in this Mazda ad. Now take the theme of the current series...
...and the PaleoZord's glowing dragon motif making it intact from Go-Onger in 'Ancient History.'
The little-known Kiwi show Maddigan's Quest, starring Rose McIver and Olivia Tennet, with a guest appearance by Milo Cawthorne, has several examples:
The show had an episode in which Rose McIver's character had to rescue Milo's and Olivia's characters from an underground toxic waste dump. Guess who rescues Ziggy and Dr. K from the cave in "If Venjix Won"?
Even earlier in that episode, Milo's character Bolek throws a tomato at Olivia's character Lilith. It seems that the two actors have experience giving each other's characters a difficult time.
In another episode earlier in the series, the traveling circus troupe stay overnight at a place called Greentown. The tunnels look exactly the same as the Alphabet Soup complex Dr. K, Gem, and Gemma grew up in. And, yes, a character played by Olivia Tennet spends some time in that complex.
Rose's character Garland falls in love with a tortured and mysterious boy from the future named Timon who, like Dillon, has been infected by the Big Bad.
A couple of Olivia Tennet examples:
Remember all of those "Odin's beard" comments from Flynn? Olivia recently appeared as a one-shot character on the Kiwi comedy The Almighty Johnsons as a potential mate to the show's main protagonist named Axl, who is the human incarnation of the Norse god Odin. Sadly, he has no beard.
In "Ranger Blue", Dr. K tells Flynn that if he wants justice, he should read a comic book. Now, thanks to Boom! Studios' Power Rangers comic book series and its Shattered Grid storyline spanning every generation of Rangers, she now appears on a cover of a comic book◊!
Also, producer Eddie Guzelian was dismayed when he pitched his "set after the apocalypse" idea— dark and action-oriented, only to find he had to adapt a bright, cutesy Sentai series around that. He had, only two years prior, done the same thing to a certain Disney film.
If the pool stick in the "Dome Dolls" scene were a little bit more to the left...
Dillon and Ziggy.
Hype Backlash: The series is revered as one of Power Ranger's greatest seasons ever, so its not unheard of to have fans disappointed. Some fans (though a minority) also felt that it didn't feel like a Power Rangers season.
Scapegoat Creator: Judd Lynn, coming in to replace Eddie Guzelian midway through the season. Even before the episodes he actually created had aired, some fans had him pegged for stuff he could possibly do to screw up the season.
Ship Tease: Summer and Dillon, Summer and Scott, K and Ziggy, Flynn and Gemma. Really, Gem was the only one out of the loop.
The season opens with the entire world being destroyed by a sentient computer virus and the last major battle in a Robot War. Although other planets may have been destroyed by villains in past seasons, the near total destruction of Earth has never happened before on Power Rangers.
Then, in "Ranger Green", we find out why Ziggy has a problem with the Scorpion Cartel: He took a bus filled with black market medicine and, instead of driving it to the designated location, he risked his life by delivering the supplies to an orphanage of children struck with gamma sickness.
Then, in "Ranger Red" and the "Ranger Yellow" two-parter, we find out that Scott and Summer lost people close to them in the war. And they don't come back.
In perhaps the biggest example, in "Doctor K", we find out that Dr. K, the creator of the Ranger Series technology, was captured at a young age by a government organization named Alphabet Soup, and was made to work on experiments for many years. Then, we find out that she was the one who programmed Venjix in the first place while she worked in Alphabet Soup. And she released it when she tried to escape captivity, along with her friends Gem and Gemma. Worse yet, Alphabet Soup did absolutely nothing to stop it and contributed to its spread by not allowing her to install the firewall that would have kept Venjix inside the compound. The handlers even tried to assassinate her when she tried to escape again after Venjix forces began attacking in order to cover up Venjix's origins.
Then, in "Blitz", Dr. K mentions to Dillon and Summer that the Sat-Bot triggered an initiation code which activated the Venjix virus previously dormant inside of his robotic parts. It is slowly taking over, and there is no way to reverse it.
Then, in "Brother's Keeper", Dr. K enters the base code for the Venjix virus before Dillon falls completely to its control. Then, to everyone's horror, she drops the bombshell revelation that she was the one who wrote and released Venjix.
Post-"Belly of the Beast", Dillon gets flashbacks of his time as a Venjix captive, including one in which he woke up in an operating table next to his sister, along with a dozen or so other people—all of them having received robotic upgrades. Human experimentation. On Power Rangers.
Then, in "End Game", we find out Venjix's true plans: Remember all of those attack bots which attacked Corinth? All just a distraction. Venjix has been sending hybrids into Corinth. In fact, over half of Corinth is comprised of hybrids. Then, Tenaya 15 activates them, allowing Venjix to walk in and take over Corinth.
Finally, in the "Danger and Destiny" two-parter finale, Venjix "deletes" Gem and Gemma . . . only for Dr. K to gain them back later, thus letting them drop a giant building on top of Venjix in the second part's climax. At the end of the episode, we see a glowing red light as a familiar tune plays in the background...
Spiritual Adaptation: As a series with a sentient machine as the Big Bad, and with one of the central characters as a black-clad, brunette with robotic parts tied to the Big Bad (though a cyborg instead of an android), this is basically the Power Rangers equivalent to Terminator.
Take That, Scrappy!: When exploring ruins, Tenaya 7 finds a battered Operation Overdrive Red Ranger helmet and, without too much thought, tosses it aside. Now, this can be seen as a Continuity Nod, showing what happened to other Rangers who tried to stop Venjix. But you gotta wonder if this was deliberate: out of all the 15 previous Power Rangers teams available, the one they explicitly showed to have died was the one from the most reviled season of the franchise.
Dillon had his Venjix virus surpassing 50% of infection level and we didn't get a Black vs. Red (or Yellow, or Green) battle.
Several later Go-Onger monsters were completely unused, likely due to the show's small budget.
The final battle of Go-Onger was pretty dang epic for how comical the series was. Venjix doesn't even grow himself to giant size at all and gets blown up by a building getting dropped on his head. (This probably had something to do with the aforementioned budget issues.)
The Un-Twist: It took twenty-five out of thirty-two episodes to confirm that Tenaya is Dillon's sister. Everyone Knew It. No, Everyone knew it...The fact that this particular subplot was done before made the whole thing completely obvious.
The Un-Reveal: Until then, most obviously in "Prisoners", the series seemed to be playing keep away with the secret.
Sad thing is this would of been Subverted if not for Eddie Guzelian being fired. The original plan was that Dillon was a sleeper agent meant to be Venjix's final body and that the sister story was a false memory meant to gain the other Rangers' trust.
Villain Decay: When Kilobyte was first introduced his original deep voice made him sound dangerous and foreboding. Then later his voice changed for no apparent reason and became just a generic British accented voice which made him half as intimating. Although some did prefer the new voice.
Vindicated by History: If there were any ranger fans who were upset by how RPM turned out, they were probably swayed by how awesome it looks in comparison to Samurai and Megaforce.
What Do You Mean, It's for Kids?: This series begins with a RobotApocalypse that wipes humanity out down to 1 city. That already sets itself apart from most Power Rangers series. It only gets worse from there, featuring blockade running, organized crime even in the face of the apocalypse, side characters dying off, humans losing their free will and identity because of a rogue virus, a Woobie, Destroyer of Worlds who destroyed the world on accident, rediscovering humanity after being forcefully converted into a cyborg, and even government oppression in the backstory. This season was initially pitched as something aiming for a slightly older audience than before, and it shows.