#7. Exploit Customization Systems

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Above, Cube World player/NPC races; NPCs are generated (unique)

Many games offer detailed player customization options that are often only used once, during the creation of the initial player character.  This is both limiting and wasteful.  The same system that players use to create unique, identifiable characters can be harnessed to generate unique, diverse NPCs and enemies.  Apart from a slim chance of encountering the occasional doppelganger, this generation allows for dozens, if not hundreds, of different NPCs/enemies (the more options, the better).
It also allows for players to create back-stories for enemies and NPCs in real-time, making your lore appear even richer, e.g., "That troll was mocked for his naturally pink hair as a child, and he swore to prove his worth by slaying twice as many adventurers as the other trolls in his tribe."

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#6. Environmental Narratives

NPCs should have purpose. Not just "a purpose".  Purpose.

In Cube World, groups of 2-4 NPCs, consisting of every race, both hostile and friendly, can be found roaming the world, doing battle with other NPCs/players.  This obviously makes the game world feel more alive.
But environmental narratives aren't limited to RPGs.  They've been deployed in a vastly different sort of open-world game, Saints Row II.  SR II uses environmental narratives quite well, both when it allows players to eavesdrop on the conversations of NPC couples, and when a hotdog-stand mascot deals roughly with a dissatisfied customer.
(See 10:35-11:58, here: http://youtu.be/BSgvVcB0mQU?t=10m35s ;)

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#5. Enhance World Borders

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Above, the ever-barred Elven city of Prifddinas (Runescape)

By hinting at content in the pipeline, game worlds can seem far richer than they are.
By showing players the entrances to forbidden cities, quarantined settlements, ancient ruins, shipwrecks, and the like, developers can create mystique while fueling speculation about the inaccessible areas (But don't overdo it. No one likes constant dead ends). 

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The Core, a mysterious entity that powers the dungeons of Spiral Knights.

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#4. Add mini-games and challenges/achievements

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Above, the first mini-game to be added to Club Penguin

Mini-games, especially multiplayer ones, can make a game that would otherwise seem lacking in content appear rich with activity.  Mini-games need to have maximum replay value, as adding high-quality disposable content (quests) necessitates the addition of more disposable content.  
Combat scenarios can make great mini-games, e.g., Guild Wars, a PvP concept that's self-explanatory, and of course the classic PvE monster arena.
Diegetic (contextual) mini-games are best, but, in a pinch, even games that are only loosely connected to the main game will suffice.

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Above, Achievement Unlocked, a game that satirizes achievements

Achievements ensure that players experience existing content completely.  To encourage players to complete achievements, a small incentive, e.g., a visible indicator of completion to the player and other players (a badge) should be awarded.

Unlike achievements, challenges are frequently refreshed objectives, with the express goal of challenging the player and rewarding them for the completion of said challenges.

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#3. Decorative/Promotional Items and Pets

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Arguably the most superficial type of content, both decorative/promotional items and pets serve to motivate players to continue playing in order to earn sufficient in-game currency to afford them (or, optionally, purchase them with real-world money).  Needless to say, the addition of this type of superficial content can also assist developers, by allowing them more time to develop richer content (a game can't be purely superficial content).

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#2. Procedural Generation

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Isn't Minecraft a gorgeous game?

An infinite world means infinite possibilities.
Procedural generation allows for an extraordinary amount of depth, as evidenced by both Cube World and Minecraft.  Even if a world generator consists of just a handful of biomes, monsters, and terrain types, it can generate a world where no two areas look the same.  Adding content to a world generator makes said world exponentially richer, as it isn't merely adding one castle to a static world, it's adding the potential for many distinct castles throughout the world.  And procedural generation isn't limited to worlds.  Quests are the holy grail of procedural generation.  Procedural quests are generated tasks with varying creatures, stories and areas.  Truly infinite content.

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#1. Make a Platform

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Don't have enough content?  Let players create it for you!
Provide great tools to create, whether it's a track editor for a racing game, or a dedicated mod API for an open world sandbox game, and reward players who use them effectively.
Support modders and your game's community!  Even something as seemingly superficial as player-owned housing and shops can make a big difference.
Last but not least, if you don't have enough content, your players need to know that you will add more.

Agree?  Disagree?  Am I completely misguided?   Comment!
I love constructive criticism.  And yes, this post is in the Internet Standard List Article Format.

Aside: This is my first post on Gamasutra. :)