In the video game adaptation of Power Rangers Samurai, the first level ends with them making the Megazord...then you get text saying that the Megazord won. No fight, no cutscene, no nothing. At least you get to fight with the Megazord in future levels (as a Quick Time Event).
After spending hours in Xenogears undergoing hideous trials such as Babel Tower and Kislev's Absurdly Spacious Sewer, it is difficult to convey how disappointing it is to have entire dungeons and some truly epic-sounding battles narrated as recaps by characters sitting in chairs.
Considering Xenogears had some nightmarish 'dungeons', especially Nortune's Goddamn Sewer, that's something of a blessing.
Not to mention Solid Snake, as a - well, as a concept. We get to play as him for two hours, if you play slowly and get all the Easter Eggs, And Now for Someone Completely Different! He runs around the Big Shell killing sentries left and right and doing crazy stunts to defuse bombs, and we have his word to go on, only. He goes off to have a showdown with a ninja, and the only indication of this we get is him telling Raiden that he has his hands full with something on his end, followed by another character explaining what had happened, later on. The raised middle finger of this whole aspect was very, very, very intentional.
Also, the opening cutscene of MGS2 explains that Solid Snake and Otacon traveled around the world for two years destroying Metal Gears as part of the non-government group Philanthropy. Considering that Snake has only starred in two full next-gen games (to date), was it too much to ask for a interquel game detailing the intervening years of their involvement with the group?
Fortune drags him atop Arsenal Gear in handcuffs. How did that happen? It was probably awesome. We probably would have liked to see that.
After Sons of Liberty Raiden goes on a journey to rescue a child named Sunny from The Patriots. With help from another party he finds her and leaves her under Otacon and Snake's care. He still has a horrible confrontation with the Patriots who play with him and ruin his body. Raiden manages to escape and journey has affected him so much that he Took a Level in Badass after being The Scrappy. He later goes on a journey to find the corpse of Big Boss in compensation assistance with Sunny. When will Kojima Productions develop a game focused on this? (The answer was original supposed to be Metal Gear Rising: Revengeance, but when they handed it over to Platinum they made it a sequel) .
In Halo, the Battle of Earth as a whole is this trope. The actual massive battle takes place in between Halo 2 and Halo 3. Similarly, a lot of the massive battles of Reach happen in the background.
House of the Dead: Overkill plays this for laughs at the final boss. Then subverts it because you fight the final boss anyway.
Agent G: Varla, this is for you!
G: How we survived that, I don't know.
Washington: Hey, if we hadn't found these miniguns just lyin' around, we'd be fuckin' dead for sure!
In Dragon Age: Origins, the enchanter Sandal apparently slaughters an entire platoon of darkspawn. You arrive in the aftermath of this.
Sandal: Enchantment? You: What happened here? Sandal:Enchantment!
The first time, being much like the event in the first game, there's a giant frozen ogre, and Sandal's response?
Sandal: NOT Enchanment...
The fangame Paper Mario World sets up a boss battle... then not delivering any boss battle at all. To elaborate, Mario's allies just managed to free Mario from being trapped in a cave, but then Bowser shows up. All of them prepare for what what would be a third Bowser battle, but then a white screen shows up saying something like, "And then Mario and friends defeated Bowser." Then you see Bowser running away, and the others continue the game as if nothing happened.
In Assassin's Creed: Brotherhood: Ezio Auditore walks into the Rosa expecting to find his sister dead at the hands of the guards. Instead he finds that she killed a group of armed and armored guards with a knife, while wearing a dress, off screen.
In the "Fate" route of Fate/stay night, Archer's standoff with Berserker is relegated to an off-screen bang, and Rin and Shirou's reactions. The anime fully renders the awesomeness, with perhaps more justice than the Visual Novel could.
Another one in the "Heaven's Feel" route, if you decide to let Rin kill Sakura. Kotomine predicts what's going to happen next and it sounds awesome - then you get a Game Over screen.
In Golden Sun: Dark Dawn this comes at the very end of the game when Alex goes off to fight Blados and Chalis off screen to give you time to save the world. Of course they all wind up surviving this little encounter. Camelot is quite skilled at never letting you know just how powerful this particular character is.
Although, it's thought pretty widely that the boss fights were supposed to be played, but got Dummied Out.
In Postal 2: Apocalypse Weekend, there is a segment where the Postal Dude is asked to help "relocate pigeons," and he is handed a rocket launcher. The game immediately cuts to a video of Running With Scissors' CEO complaining over the phone that they don't have the money to make such a level. When the game cuts back to the Dude, he's standing in a massive pool of blood and feathers.
Dude: "Man, that was the most incredible thing I've ever done!"
Runescape features a quest called King of the Dwarves. Here's the rundown: the Big Bad is speaking to a mob demanding that the oligarchs who run the city reinstate the monarchy based on records of who is descended from the king. Meanwhile, his forces (only the character knows they are his anyway) are preparing an attack on the city to divide the guards. Your character and friends sneak into the record vaults and discover that said Big Bad is the rightful heir. The historical expert stays behind to forge the documents to keep Hreidmar from taking over while you and Veldaban go to help the city guards against his forces. You do so by tricking a troll warlord into doing it for you. And what does this epic battle look like? The troll warlord tells you to go back to the city and he'll take care of it.
Sam: And then there was the most epic battle of our entire career!
In Mass Effect 3, after spending the entire game gathering war assets and building an army, almost none of your war assets are seen or even mentioned during the final battle. This is one reason why the ending is so widely despised.
More generally, from the first game, it's been established that the Elcor fight by strapping massive clusters of heavy weaponry to their backs, and laying waste to their enemies using VI-targeted fire. They are referred to on several occasions as "living tanks". After three games, we still never got to actually see this.
We hear two different, unrelated stories about a Krogan battlemaster and an Asari commando tearing apart a space station in epic duels. Both times, the storyteller neglects to describe any parts of the battle itself.
One minor Fetch Quest involves finding a dinosaur fossil so they can be cloned and used as cavalry. As with all other War Assets, we never see the results of this.
Pretty much everything Anderson does: Earth is one of the first planets attacked, Humanity's homeworld takes the brunt of the Reapers' wrath, its main cities are destroyed within days if not hours, yet Anderson manages to mount a resistance movement, reorganize the Humans living outside the cities efficiently enough that not only does Earth resist until the very end of the game after Thessia has gone silent and Palaven has been abandonned by Turians who cannot both defend their homeworld and prepare to escort the Crucible anymore, but manage, with no help from the outside to prepare a final assault against the Reaper beam in London. And we don't see any of it.
The Miracle of Palaven also qualifies. The Turian navy draws the Reapers away from Palaven, the Turian leadership and resistance infiltrate the Reapers when called aboard them for "peace talks" and set up bombs, destroying several Reapers from the inside. At the same time, the Krogan (with said dinosaurs) arrive and join the Turian military in a concerted push that takes back large areas and boost galactic morale. Sadly, we only learn about it from a Codex entry.
Many Companion quests in Star Wars: The Old Republic involve some of these (as companions after the first accomplish most of their personal goals off screen to save development time. Jaessa Wilsaam kills a sith lord in single combat, despite only being an apprentice herself, Lt. Pierce manages to take one of the most fortified stations in history with only a small team of black ops troopers and a small detachment of soldiers, while M1-4X's entire storyline involves ever increasingly awesome deeds of patriotic fervor, starting with capturing an Imperial propaganda specialist through incredibly precise timing, a flimsy window, and a passing speeder, and ending with Capturing an Imperial scientist... with only basic motor functions intact.
Street Fighter has Akuma's Raging Demon attack. It is never shown onscreen, but it is one of the most epic and powerful moves in the entire series.
"Die one thousand deaths!"
The Umbrella corporation in the Resident Evil franchise is portrayed as the evil Mega Corp. that caused the zombie outbreak throughout multiple games. The endings of Resident Evil 2 and Resident Evil 3: Nemesis imply that the protagonists are going to take on Umbrella directly, but the closest they ever gotten was Claire storming Umbrella's headquarters at the start of Resident Evil: Code: Veronica and then getting captured quickly. By the start of Resident Evil 4, Umbrella gets into trouble with the government and they fall into bankruptcy. To rub the salt further into the wound, Jill and Chris in a flashback during Resident Evil 5 are shown to be hunting down the man behind Umbrella, but Wesker intervenes and cuts their plan short. A shame that the player never gets to take Umbrella down themselves directly. Spencer, the guy behind Umberlla, is shown to be an extremely frail and old man confined to a wheelchair and is killed off by Wesker in a cut scene.
Team Skull attacks Wigglytuff and attempts to knock him out. This fails, and Wigglytuff singlehandedly destroys all three of them without breaking a sweat off screen. Team Skull is not seen again for a long time.
Wigglytuff has another one when narrating his flashback of the first visit to Brine Cave; Chatot prevents Kabutops and his henchmen from instantly defeating Wigglytuff after which Wigglytuff effortlessley thrashed them.
The Elder Scrolls V: Skyrim has a moment where the Dovahkiin has to ride a dragon to reach their final destination. "Hold on, Dovahkiin! Prepare to see Skyrim as you never have before!". As soon as they rise into the air and the choir kicks in, the player is skipped directly towards the destination. They adressed this later with the "Dovahkiin" Downloadable Content, which adds the ability to travel Skyrim by riding dragons.
Super Smash Bros. Wii U/3DS has the reveal trailer of Lucina and Robin, which starts off with Chrom (also from Awakening) lying in defeat and Lucina barely holding her own after facing Captain Falcon in battle, who looks no worse for wear. There's still plenty of awesome afterwards though.