These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Character Tiers: There are no levels, so the Marshall is on his own when it comes to scaling opponents to the levels. All of the major villains also form a God Tier. They originally didn't even have stats, and even when they were eventually statted they all have Contractual Boss Immunity.
Complete Monster: Jasper Stone is the Harrowed responsible for bringing Hell to earth. Originally a Confederate officer with a reputation for cruelty and brutality. When he became the servant of the Reckoners, the Four Horsemen of the Apocaypse, Stone ruthlessly killed any who could stand in their way, hunting down and murdering countless heroes. Stone does everything of his own free will for his hunger to take lives and to see hell on earth enacted.
Creator's Pet: The Classic supplement Black Circle introduced "a major new force in the Weird West: the Cackler". The authors built up the Cackler as a great mystery and all that readers really knew about him was that he was supposedly the most evil Big Bad ever to walk the world. Heck, the authors even told Marshals not to use him in the game because they had a huge plans for him and wanted to have full control over how he was used. Actual quote: "What you should not do is bring the Cackler into your game yet. He’s coming, and you’ll know it when he does. We’re not ready to say exactly when, but his presence will change the Weird West forever."
For what it's worth, Pinnacle has since done a complete 180 on this, saying that it's highly unlikely that The Cackler will ever turn up again in a Deadlands RPG book. Shane Lacy Hensley has said that perhaps he'll tell the Cackler's story in some other media, but it's clear that they won't bring him up in a future supplement (it's hinted that the story wouldn't really have anything to do with a typical posse).
And indeed, the Cackler's story is going to be revealed in 2015 as a graphic novel, rather than a tabletop supplement.
To a lesser extent, the Four Servitors were Creator's pets as well given how much was written about them and how far the authors went to make sure they were overly powerful and unkillable. Heck, a few scenarios involved the heroes unwittingly making those characters even more powerful (sometimes without them even knowing it)! By the time that Deadlands Reloaded came around, the authors generally relegated them to the background and commissioned a comic series where each of their backstories (which again, had little to do with a posse) could be properly explored.
Genius Bonus: As noted in the corebook, "Harrowed" means "Dragged (forth) from the earth". The word is used as the terminology for sentient undead.