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.
YMMV: Kerbal Space Program
Internet Backdraft: The community is generally a pretty laid-back bunch, but there are two particular topics that tend to be the subject of intense debate whenever they're brought up: Multiplayer (both how to implement it and whether it should be implemented at all) and autopilot add-ons (specifically whether they're helpful learning tools for new players, crutches that take away the fun of learning to pilot manually, or something that takes away the tedium of flying manually when you've done it so many times already).
A new classic is now "Hey, how about Lagrange points?" along with a proposal on how to implement them that's already been discussed and dismissed a hundred times.
Never bring up the topic of "female Kerbals" in any context whatsoever. Just don't.
Memetic Badass: Nothing scares Jebediah Kerman, aka "the Thrillmaster!"
Good Bad Bugs: The aerodynamics are a bit primitive at this stage, allowing for some wonky airplanes. For example, you can make one that is powered solely by flapping control surfaces. You can also make single-engine, air-breathing planes that can achieve orbit by fitting them with enough intakes. Said intakes do not even have to have a logical connection to the engine. They just have to be on the plane.
You can destroy planets with with struts! (Unfortunately fixed since that video's release.)
That One Level: While every planet and moon has its own unique challenges, a few of them have gained a certain amount of notoriety among the community:
Eve is probably the most infamous. It's deceptively easy to reach the planet, and its thick atmosphere allows you to land anything on it with parachutes using minimal effort. Returning from the surface, however, is a completely different story. High gravity and the same incredibly thick atmosphere which slowed your descent will also fight against your climbing rocket every step of the way for almost 100 kilometers, creating incredible amounts of drag and killing your engine's efficiency ratings. You can mitigate this a little by launching from a higher elevation, but designing a rocket capable of landing on and returning from Eve is still an incredible test of your engineering skills. Spaceplanes apparently have a slightly easier time of it, but those are a pretty good candidate for this trope all by themselves.
Tylo is a moon of Jool with a size and surface gravity roughly equivalent to that of Kerbin, but no atmosphere. Without an atmosphere to slow you down, you'll have to use your engines to completely kill your velocity in order to land safely, which will take a lot of fuel and effort. On the plus side, that same lack of atmosphere makes it relatively easier to reach a stable orbit... unless you burned too much fuel on the landing, which you almost certainly will the first time you try.
Moho is a large, airless rock in a noticeably eccentric orbit close to the local sun. All of this conspires to make it one of the more difficult planets to reach, land on, and return from. If you hit it at the lowest point in its orbit, you can easily zip right past it before you can lower your orbital speed enough to let it capture you.