A new age is upon Britannia. The great evil Lords are gone but our people lack direction and purpose in their lives. A champion of Virtue is called for. Thou may be this champion, but only time shall tell.
Absurdly Low Level Cap: You may start the game anywhere from level one to three (depending on your class) and can progress up to a whopping level eight. Of course, this is the game where the Karma Meter matters a lot more than the Experience Meter, and don't assume that getting even to this level cap is going to be easy.
Always Chaotic Evil: Most (but not all) of the creatures you will encounter in the wilderness and dungeons are this. Feel free slaughtering the evil creatures, but let the non-evil ones flee.
Sadly, played straight in the NES version. No monster you encounter in the game will run from you, forcing you to kill any monster you randomly encounter.
Arbitrary Headcount Limit: You can have one party member for each level you gain (meaning that at level 1 you only have one slot—for yourself). And the level cap is eight.
Arc Number: 8. There are eight virtues with eight shrines dedicated to them, eight character classes represented by eight recruitable companions (although there are only eight slots in the Player Party, including one for the Avatar), eight major towns in Britannia, and eight dungeons with eight levels each.
Artifact of Doom: Mondain's Skull (remember, the villain from Ultima I) Using it would instantly wipe out every living thing in the player's current location, except for Lord British (even Lord British in the NES version). It also drove your Karma Meter (all of them!) to 0. Since many players at the time didn't realize there was a Karma Meter...
Broad Strokes: Looking back at the first three games, the events of them weren't...quite as described by Ultima IV. Time Travel may explain some of it, but by this time the general rules and definitions have solidified.
Dialogue Tree: One of the earliest examples of the concept; in PC versions of Ultima IV one could have limited conversations with everyone in the towns. Later games would make it easier to come up with topics; this one made you type each one out. "Name" and "Job" were the common opening lines; and then later Arc Words tended to be important. The only three words that all NPCs were guaranteed to respond to were "Name" "Job" and "Health". Occasionally an NPC in their dialog would let slip a subject that you could then bring up to another NPC. Since this could lead to Sequence Breaking, later games made them spell out the conversation topics depending on what information flags you encountered.
Doing in the Wizard / Doing in the Scientist: The elves, dwarves, halflings, and Ewoks that were playable races in the previous three games have been retconned out of existence, as have the spaceships, time travel, and other anachronistic elements.
Dungeon: There is still fighting in this game; and 8 dungeons with various MacGuffin.
Dungeon Bypass: The white stone is hidden up in the mountains north of Britain and to get to it you need to descend to the bottom level of a dungeon via a hidden entrance behind Lord British's castle, climb your way back up to an exit on the other side of the world, board a hot air balloon, and guide yourself to a landing space the size of a single tile with the wind-change spell. You can also just use the Blink spell (warp a distance on the world map) in the right spot and warp to it.
Egopolis: After uniting all of Sosaria, Lord British renames the world Britannia. A rare example when it's a good guy who does this.
Embodiment Of Vice: There are eight sins or vices that are the opposite of the eight Virtues. While the Virtues are represented by seven shrines throughout the land and the eighth in the Ethereal Void, the Vices are represented by seven dungeons and the eighth in the vast underworld.
Deceit is the dungeon opposing Honesty
Despise is the dungeon opposing Compassion
Destard is the dungeon opposing Valor
Wrong is the dungeon opposing Justice
Covetous is the dungeon opposing Sacrifice
Shame is the dungeon opposing Honor
Hythloth is the dungeon opposing Spirituality
The Great Stygian Abyss, the game's final dungeon, opposes Humility
Embodiment Of Virtue: The main protagonist's companions, each one representing one of the Sacred Virtues of Avatarhood:
Mariah the mage represents Honesty
Iolo the bard represents Compassion
Geoffrey the fighter represents Valor
Jaana the druid represents Justice
Julia the tinker represents Sacrifice
Dupré the paladin represents Honor
Shamino the ranger represents Spirituality
Katrina the shepherd represents Humility
Fighter, Mage, Thief: The first three character classes are called Mage, Bard, and Fighter, though the Bard class doesn't have any abilities related to music, and is actually a generic RPG "thief" character in all but name. According to the Word of God, the class was called "Bard" because "Thief" implied dishonorable behavior, and therefore didn't fit the game's theme of becoming virtuous. The other classes are variants of the first three, except the Shepherd, which has the strengths of none of the other classes.
Fission Mailed: Many players Rage Quit and reloaded when their boat was sucked into the whirlpool, since the game went black and you got the same initial text that you got when you died. Except - it's one of the only ways to get to a town with a Plot Coupon. D'oh!
Functional Magic: Like traditional games, required the correct Magic Points, skill levels, and such; but also required you to have the right combination of magic ingredients. You were expected to know and remember each recipe; whether by instruction guide or NPC.
Gender Bender: For some odd reason in the NES port, Julia a woman companion, was replaced with "Julius" a burly looking male.
Genre Shift: The NES port plays much more closely to an eastern RPG than a western one, streamlining much of the gameplay and introducing Random Encounters.
The items required to complete the game can only be found through the "search" command. Their location can be hinted at by friendly characters, but otherwise the spot where the item is supposed to be appears perfectly normal and featureless (not always, though).
The dialog file of a character who was supposed to give you a hint to the final riddle was mistakenly removed from the game, leading to a number of people struggling through the game only to find themselves stuck on the last puzzle (although the answer is spelled out for you by praying at the eight shrines, which is required to complete the game). This gave rise to the Ascended Glitch character Smith the talking horse, who appeared in several later Ultima games, always giving you important hints about the *previous* game.
The High Queen: In Forever, Lord British is replaced with Lady Britishnote due to rights issues, not Rule 63.
Joke Character: If after taking the intro quiz your highest virtue is humility then you get the shepherd class. Leather armor, a marginally better weapon selection then the mage, no magic, a poor trap disarming ability (though no worse then that of over half the other classes), and you start the game at a lower level than the other classes, stranded on an island infested by demons with no shops and no clear way out.
On the other hand, there is a Magikarp Power potential here. If you know what you're doing it's fairly easy to level up quickly. Once you do master the powers of the Avatar, you can equip anything and cast any spell; even if you're a Shepherd. Since you still need each of the other classes in your party to get to the bottom of the Abyss, you no longer have a useless party member.
Karma Meter: The Trope Namer. "Karma" was how well the character acted in the 8 Virtues. Honesty, Compassion, Valor, Sacrifice, Justice, Honor, Spirituality, and Humility. Each of the 8 were determined by actions you made. Run from a fight, and lose points in Valor. Unless you're running from a natural animal who is just hungry; such as a common serpent; and then you're adding Compassion. It gets complicated.
Lighter and Softer: According to The Official Book of Ultima. The concept for the game came about when Garriott noted Moral Guardians' response to the first 3 games. He realized that while they were wrong for the most part, the Strawman Has a Point, and so the concept for the fourth game was born. This is one of the few examples of this trope where the series actually gained more depth and character because of it, rather than the opposite.
Merging The Branches: You have eight possible party members but you can never recruit the one of the same class as yourself. Nevertheless, all eight are regarded the Companions of the Avatar in later installments.
Monster Town: Magincia has become this since its location has become so uninhabitable that every human except for Katrina is either dead or has moved away. The monsters will only attack you if you attack them first, though. ...except for Nate the Snake.
Nintendo Hard: Of all the playtesters, only Richard Garriott himself actually finished the game before it was released. If you can complete the game without using internet spoilers, you deserve a medal. Also, ironically, the version released on the NES itself is a lot easier than the original.
No Antagonist: Your quest is simply to prove yourself worthy of each of the eight virtues, something that's actually a lot harder without a villain to contrast yourself against.
The Paragon: The main reason you have to become the Avatar is so you can serve as an example for the rest of the world to follow by living virtuously.
Player Personality Quiz: Possible Trope Maker. Unlike the previous games where you specified your characters' class and stats directly, you now have to answer seven simple questions on how you would act in certain dilemmas. This determines your character class, starting location and your initial standing on the game's eight Karma Meters.
Random Encounter: The Nes version does this in a sneaky way. As they aren't step based, they are time based. Meaning wait long enough on the overworld map in one spot a battle will start. Averted in the PC version as the encounters are not random at all.
Read the Freaking Manual: The game directly tells you to read the included The History of Britannia in the intro (you should read The Book of Mystic Wisdom as well, despite your character not daring to open it).