My favorite RPGs are those that can be played with other players. This doesn't only include RPGs that have multiplayer modes, but also those that generate a lot of discussions. What makes "community RPGs" hard to come across is the harsh truth that they need to be popular enough to create a community around them. However, there is more to a community RPG than being popular, a fact which is perfectly captured by the GameFAQs top 10 games versus the top 10 message boards. Even though there is some overlap between these lists, the differences show that the games people want to discuss aren't always the most popular games.
One of the qualities of community RPGs is immersive worlds that the player can imagine themselves in. A protagonist that doesn't have a personality or plot centered around them is a better avatar for community RPGs because they are nothing more than a stand-in for the player. It is also essential for the world to feel like it could be inhabited by other players. By alluding to characters that are on the same quest as the protagonist, it makes the idea of other players inhabiting the world more reasonable. Pokémon is a great example of this because not only is the protagonist mute but the world is also populated with other Pokémon trainers. Creating a world that is believable through world-building is another aspect that is crucial for creating an immersive world. One method of doing this is world-building through theming, like how Pokémon and Boktai both add prefixes ("Poké" and "Solar," respectively) to facilities and devices to make the world feel consistent with the elements that make it different from reality.
Player expression is another aspect of community RPGs and usually manifests as collectibles that are the focal point of the game system. These collectibles can come in many forms (monsters, cards, machine parts, etc.) but the important part is how much they allow the player to customize their player build. The higher the level of customization, the more the player can express themselves and the easier it is to create a unique player build. Offering many ways to obtain these collectibles (exploration, shopping, crafting, etc.) is crucial because it ensures that players that play differently will have different collectibles. The collectibles should also have a layer of mystery surrounding them because that will encourage players to try and discuss different methods to obtain them. Dragon Warrior Monsters accomplishes this with its robust breeding system that combines two monsters into a rarer one. While Pokémon has whole urban legends that have started because of rare monsters like the elusive Mew.
Now, by playing together I don't necessarily mean multiplayer modes, but it does include them. One interesting way to play together is the Azure Sky Tower in Boktai: The Sun is in Your Hand. The Azure Sky Tower is a 99-floor dungeon that bestows each player with one out of four emblems. The only way to complete the Azure Sky Tower is by collecting all the emblems which can only be done by swapping emblems with other players through the "emblem resonance" feature. Another interesting way to play together is the "brotherband" feature in Mega Man Star Force that grants Mega Man abilities from the players he forms brotherbands with. In fact, forming brotherbands by sharing friend codes with other players was the reason I joined the GameFAQs forums. However, in my opinion, the best way to play with others is multiplayer modes that have options for both competitive and cooperative play. Pokémon famously allows players to battle and trade monsters with each other, while the summoning and invasion features in Dark Souls let you enter another player's game to either help or impede their progress.