The key concept to understand the difference between RPG's fiction and written fiction is that of authority.
In a written story, trivially the author has authority over main characters and the environment, so she can optimize the sequence of events (the plot) to heighten the emotional impact for the reader (if she knows what she is doing).
But let's not forget that RPGs were invented to mix the experience of war games with the emotional appeal of written fiction.
In war games, the environment is fixed, and each player has authority over one and only one character (whether an army or a single person doesn't matter). To mix this with the kind of flexibility the story in a book can have, the most natural step is to simply give authority over environment to another player.
Thus the concept of Game Master is born: it's the single person who has the responsibility of setting-up a situation, gather the reactions of other players, and let the game mechanics decide the result.
There is a problem though: since dice have no sense of aesthetics, it can often be the case that a satisfactory resolution of a conflict is spoiled because excessive bad or good luck. The Game Master is then given authority over rules, so she can bend or ignore them for the sake of the story. This is the modern understanding of the GM: the player with authority over rules and environment, with the implicit responsibility of the plot.
However, once you explicitly recognize this, you can start playing with the structure.
You can, for example, remove the authority over rules, keep the GM's authority over environment, and come up with game mechanics that automatically steers the story in an interesting and balanced direction. Games like Dogs in the vineyard or Apocalypse world retain a traditional role for the GM (minus rule-bending) while employing narrative conflict resolution (the so-called narrative games).
Or you can have chance-based conflict resolution but share the authority over environment and characters between active and non-active players, usually in a turn-based structure (the so-called master-less games). If I'm not mistaken, Polaris, Shock and Dirty secrets follow this structure.
Other games employ both: Fiasco, for example, has a turn-based shared authority with a bare-bone narrative conflict resolution; it is thus in the category of master-less narrative games.
All these approaches produce different kind of stories, aimed at different kind of emotional impact: written fiction is (supposedly) maximized for the passive consumer, who has no responsibility. RPGs, on the other side, optimize for immersion and responsibility, and regulate the unfolding of the plot by mechanic means, sharing authorities in different ways.
This blog defines a plot hook as:
an in-game element that inspires a strong motivation to pursue a course of action that furthers the plot or enriches a narrative in a game.
Which sounds about right to me.
All of what you listed are plot hooks. They are in-game elements (events, info, whatever) that inspire (or at least, attempt to) the PCs to take some action to keep the game narrative going.
- Nearby villages have been struck with plague - Depending on the party, they'll either want to avoid the area or try and find a cure. Either way leads to plot development. Quest for a cure, or plague continuing to spread and cause problems.
- A shipwreck swarming with orc raiders - Orcs are (usually) evil (and are worth XP). And the ship is probably full of treasure. Let's go get them!
- Two paladins are fighting a duel of honour - What would cause two icons of Good to fight? Some sort of evil magic, or just a severe difference of opinion? Sounds like a mystery to be solved.
- A cursed Wand of Wonder - All sorts of random chaos to make the party's lives "interesting". This is probably the least plot-hook-y. But it may inspire the PCs to try and remove the curse, which could lead to a quest.
- You're invited to your rich friend's estate (where you'll just happen to meet the rest of the PCs) - Your character now has a reason to be in the same location as the rest of the party, so they can join up (and then another plot hook can get them going on an adventure).
Best Answer
You're right, these three things are key to a true West Marches campaign:
Ben Robbins, whose blog you quoted, should know, he popularised the concept!
There is a bit more to it though...
Implied, but not explicit in those first three points:
Every game session begins and ends at the same point of origin (normally a home town). This means:
The players decide where to go and what to do in advance.
Outside of those first three points:
Session reports are always shared
There is a shared world map, that's potentially unreliable
All initial objectives and later objectives that are discovered are marked on a shared map, which players can use to suggest places they want to explore.
The initial map is produced in-game by a character and is only as reliable as that characters map-making abilities / trustworthiness. It is later edited by the players who may also make mistakes. This means it's possible to get lost, if the map is wrong (and the players can subsequently correct it).
Competition between players is actively encouraged
Content is loosely tiered
Matt Colville made a great video that covers all of this in detail, if you don't fancy reading all of Ben Robbins' blogs.
However, when someone says they are running a "West Marches style" campaign, they may often mean something much looser than this:
The only conclusive way to know what someone else really means when they say they're running a "West Marches style" campaign is to ask them.
That said, at a bare minimum, it is likely that these things are implied:
They don't have a regular group of players, but pull from a larger pool each session.
Each session will be entirely self contained.
There's probably a focus on exploration.