Undying Loyalty: The true geth. No matter how badly they were treated by the quarians, even to the point of genocide, they still value them as parents. Given the chance, the geth repay the quarians in a big way, even restoring their immune systems and caretaking their world. There's a certain amount of pain from Legion when he ponders what else the geth could do when their own beloved creators wanted to destroy them.
Avertedtragically when the geth find themselves getting owned by the quarians after Admiral Xen invents a weapon that works as a win button, and their only chance for survival is to ally with the Reapers. Legion is visibly ashamed at this reaction by his people.
Unexplained Recovery: The former trope name (I Got Better) is invoked by the Commander in the second game.
Unholy Nuke: When possessing a puppet, the Reaper Harbinger has a special version of of the "Warp" ability that qualifies.
An ability also possessed by Banshees in the third game.
As of the last multiplayer DLC, the playable Collector has this same ability.
Unobtanium: Element zero is stated in the Codex to be generated only when stars go boom. The primary sources of eezo are, thus, the debris fields surrounding neutron stars and black holes, requiring extensive set-up costs for the telerobotic systems and radiation shielding necessary to mine it. It is also the only means of manipulating mass known. It is thus the rarest and most valuable substance in the galaxy. However, everyFTL craft, everyFlying Car, everyomni-tool and every firearm (not to mention every biotic) has a few grams somewhere inside of it. Not to mention, two words: eezo toothbrushes.
Luckily, not all that delicious Unobtanium stays in such harsh places. Good-sized chunks are often caught in the orbits of younger stars, where it can be mined much more easily; Omega Station was the result of one such mine. And the galaxy seems to be filled with Ghost Planets which gathered the material for their own use, meaning Adventurer Archaeologists can make a cute credit or so doing a little Grave Robbing. This probably destroyed most remaining evidence of The Reapers in the process. One could assume that this is the case for all Mass Effect technology; why would a centuries-old galaxy-spanning civilization that uses the stuff extensively everthrow any of it away?
Unreliable Expositor: The Codex is sometimes this, as only what is officially known and admitted is written there. In most cases the information given in the Codex is true, but in a few cases it is not, such as that the Citadel and the mass relays are built by the Protheans instead of the Reapers. Finally averted in the third game, as the Reapers make themselves known, and how.
Unusually Uninteresting Sight: Not only doesn't a trio of heavily armored and armed to the teeth soldiers turn any heads, you can discharge an automatic weapon right in the middle of the seat of the galactic government and onlookers won't even interrupt their conversation.
You can also take a geth with you to the Citadel, or a turian or salarian to Tuchanka, without even a pause from the locals.
Averted however if you take Legion the Geth Infiltrator with you to the quarian Flotilla. They will have something to say about it. Also becomes somewhat ironic, given the circumstances behind you being there in the first place.
Unwinnable by Mistake: In all three games there are dangerous spots where your character can cross some invisible boundary and become unable to go back, forever hanging in mid-air. In 2 there are such spots on the MSV Estevanico and Alchera, in 3 there is a particularly nasty spot on the Normandy's bridge.
Vicious Cycle: The Reapers wipe out all sapient organic life in the galaxy at least every fifty thousand years or so. They state that this cycle has repeated itself more times than humans can possibly imagine. So, let's just give you a brief picture: there is hard evidence that the Reapers are more than a billion years old. Not simple enough for you? Then here's the lowdown: this means that the Reapers have invaded at least TWENTY THOUSAND TIMES. And it's not even the first time that someone tried to fight the Reapers, only to be defeated.
And their purpose is to prevent yet another vicious cycle (i.e. technological singularity) from wiping out organic life full-stop.
Victory By Endurance: According to the Codex, this is humanity's main method of fighting. Humanity attacks the enemy's supplies, headquarters and resources foremost, leaving enemy forces leaderless with less logistical support until the human fleets can curb stomp them. Humanity's approach to warfare is based on rapid reaction forces which seek to decapitate command and control and avoid attrition while remaining highly manoeuvrable. Mass Effect's codices basically have humanity fighting Third and Fourth generation warfare...in SPACE! The rest of the galaxy has barely managed to catch up to human innovations in strategic thought from about the time of the biplanes. Turians still fight massive wars of attrition. Humanity is always shown as being simply more tactically proficient than everyone else to the point of actually subtly paralleling Earth history. As the turians and everyone else build dreadnoughts, humans realize carriers are the future and make many of them. Humans have the first ship able to use "stealth." It bears a striking resemblance to a submarine. Sound familiar? The turians may as well call their military groups Legions for how "groman" they are. Salarians don't have the endurance that human forces have and the asari are still fielding armies of light infantry from city-states. Krogan are shown to be poor strategists for a race of proud warrior race guys.
Video Game Cruelty Punishment: In the third game, you will have to answer for the important Renegade choices you may have previously made, and cumulatively, will have an adverse effect on the eventual outcome of some of the endings you choose. Even within the game, neglecting to do some side missions will have certain consequences as well, one of the worst being having to face your former squadmate as an enemy.
All computer screens featured in the series, as well as the loading screens from the second game, generally play this trope very straight. Most of the things listed at the trope's page - DNA helix animations on medical computers, random numbers, large fonts, and a Hacking Minigame featuring illegible bits of code - are there.
Also the Virtual Intelligences, humanoid computer programs. Bioware most likely added them so the player could interact with electronics without a bunch of reading involved.
Villain Override: First seen in the original game when Sovereign turned Saren's corpse into a super-Husk. Extremely prominent in the second game, to the point of giving the trope's page quote, with Harbinger and the Collectors, and one of the most infamous Interrupting Memes in recent memory.
Inverted when after failing its appointed task, the Reaper Harbinger releases control of the Collector General, leaving it to its fate.
Virtual Ghost: The quarians used to do this, although it was to preserve the knowledge of their ancestors and not to become immortal. They stopped once the geth conquered their homeworld and destroyed the databanks containing them.
The Protheans are particularly fond of this trope.
According to Cerberus News, a ship containing a one-billion-strong race of virtual aliens has made contact with Council space after travelling the galaxy for 8000 years after uploading their minds to a virtual world to save their civilisation from destruction.
The Virus: Again, two to four varieties, depending on your personal criteria.
We Are as Mayflies: Both the krogan and the asari live for about a thousand years, whereas humans in the 22nd century are lucky to live to one hundred and fifty. Inverted with the salarians who live to be about 40. The vorcha, introduced in Mass Effect 2, have a lifespan of only 20 years.
We Cannot Go On Without You: If Shepard is KO'd during a battle, you get the Game Over screen (unlike Shepard's teammates, who automatically recover after the fight ends if they were KO'd, even if the Unity ability isn't used).
This trope is acknowledged if you get the worst ending of the second game. Joker also mentions it early on as the reason the team broke up after Shepard was killed — they were Shepard's team. Without him/her, there was no reason for them to stay together.
Thane: Attacking the Collectors would require passing through the Omega-4 relay. No ship has ever returned from doing so. Shepard: They told me it was impossible to get to Ilos, too. Thane: A fair point. You've built a career on performing the impossible.
Welcome to Corneria: Typical BioWare approach. NPCs usually answer the same thing whenever you ask (for convenience). Subverted by/averted by some random NPCs.
Well-Intentioned Extremist: The Cerberus organization, at least if you're human. Except if you're a biotic, or any particular individual, as the survival of the species overrides anything else (ethics, human decency, etc).
At the end of ME3 we learn this of the Reapers. They were created long, long, long ago by the Leviathans, aimed to end the chaos that would end their thrall races. Not the chaos of organic evolution as implied earlier, but the chaotic destruction of organic life by synthetic life, as the Leviathan's thrall races would create synthetics to help them which in turn would wipe that race out. So when organic civilizations become advanced enough to create synthetics that begin to rise against them, the Reapers come in, immortalize the advanced civilizations as new Reapers, and destroy all civilization, then retreat to deep space to wait for it to happen again.
We Will Have Perfect Health in the Future: Medical science advancements have extended the human life expectancy to around 150 years. Some people still get diseases though. Such as your pilot, Joker, who has a rare medical condition that makes his bones really brittle. Though even that technology is advanced, since Joker said if he was born during our time, he would've died as an infant.
What Beautiful Eyes: The second game really ratchets the visual quality of all the character eyes. Miranda and Doctor Chakwas are two of the prettier examples.
Default male Shepard has striking blue eyes while on the flip side, the default female Shepard has bright green eyes...unless you go renegade.
What the Hell, Hero?: Depending on your choices there can be several such moments, but Shep gets most thoroughly called out by Anderson in the end of the first game if s/he claims that they intentionally let the council die to weaken the other races and advance humanity.
Whole Plot Reference: The third game is one protracted quest to enlist help from multiple factions who all distrust each other to some degree in order to prevent the fall of civilization at the hands of Eldritch Abominations that can only be defeated through the use of a superweapon of ancient design, all while a secondary villain stands in your way because he thinks he can defeat the Big Bad by his own means. And the hero might end up having to sacrifice him/herself in the end to finish the task I do believe BioWare has been down this road before.
A Wizard Did It: Or rather a mass effect field. Why your squadmates can survive in hard vacuum without a helmet. Probably.
Given that the squadmates who don't have helmets are nominally biotics, such as Miranda and Jack, its possible that they're simply using a biotic field to protect them.
Not to mention EVERY MAIN CHARACTER IN THE SERIES, especially Commander Shepard.
X Y Z
Xanatos Speed Chess: The Reapers have backup plans and contingencies for just about everything the protagonists throw at them. Makes sense, as they've been at this for a long time and have likely seen similar strategies in past cycles. See the Speed Chess page for a full list of their plans and adjustments they make.