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.
The Hindmost reveals to Louis Wu in The Ringworld Throne that he has a quantum computer and the nanotech-based autodoc invented by Louis' biological father Carlos aboard his ship, without having mentioned them at all in the previous book.
In Ringworld's Children, Louis discovers that Teela became a protector on one of the Maps of Pak in the Other Ocean, then traveled to the Map of Mars on the other side of the Ringworld and pretended to turn into a protector again for Bram's benefit. Apparently, the only reason for this is to justify why Wembleth lived near the Other Ocean.
Ringworld's Children also introduces the concept of monsters in hyperspace that take time to catch and eat spaceships, in order to explain how the entire Ringworld, 1 AU around its sun, can go into hyperspace without being immediately destroyed.
Canon Sue: In Ringworld's Children, Louis is transformed into a protector, but after using his new abilities to resolve the plot, he gets better thanks to Carlos Wu's autodoc. If becoming a protector is now a reversible condition, it diminishes the Blessed with Suck aspect that protectors have always had.
Inferred Holocaust: The climax of Ringworld features kind of a doozy. They leave the Ring by dragging their ship with shadow square wire. Thing is, they only have one end of the wire. The other is coiled up in a heavily inhabited city. Ever seen a high-tension cable break free? It can take off limbs. Now imagine hundreds of miles of razor sharp wire doing something similar... Apparently the author noticed this "small" problem (or a fan pointed it out) between the first book and the second, where our hero goes What The Hell, Me.
Although the way they get the end of the wire they have is by retrieving it from the death-trap it was used to set up. The locals used it to string up the shadow square wire at neck-height, behind the protagonists.
The Ringworld Throne and Ringworld's Children are arguably not as good as the first two books in the series because of the number of Ass Pulls required by the plot (see above). The Ringworld Throne also suffers from focusing primarily on the Fearless Vampire Slayers, with Louis spending most of the story only watching them over the Hindmost's video feeds.
The Ringworld Engineers is also considered by some to be a step down in quality from the original book, mainly because of the odd hoops the characters are forced through to justify their new characterization in the story.