Awesome Music: The soundtrack is pretty good overall, but the tune that plays in the Research and Development center is just the perfect tune to get one psyched up for a space flight. It sounds like something you would hear on a movie about space flights.
The tune played when you break out of atmosphere is haunting and 2001-esque.
The release of 1.0 caused a major split within the game's fanbase. The update was hyped considerably before its release, and introduced many new features and fixes, including a new aerodynamic system and the long-promised female Kerbal. Some people loved the update, but others were very vocally disappointed. They claimed that the update didn't fix enough, that it introduced new bugs, that it broke old bugs that players had come to rely on, that Valentina and the other female Kerbals were dumb, etc. Accusations of sexism flew back and forth, and things just went downhill with every patch released to fix things. Most updates cause this to some degree, because things are changed or added and most mods become incompatible for a while—and sometimes permanently—after each update.
Some of the prominent mods have caused minor schisms in the community:
MechJeb and other autopilots are the biggest one. The stock game requires you to fly all vehicles manually, but some players find this difficult and/or tedious, and want the game to do at least some of the work for them; non-autopilot users insist this makes the game too easy, or even pointless.
FAR, an advanced aerodynamics-modeling mod, is sort of the opposite: it makes the game more realistic, but also more difficult. Fans of it are often rather condescending towards the standard aerodynamics and the players who use it, while the non-FAR players think it just makes planes too difficult and rockets too easy (because it makes narrower designs more efficient). Improvements to the base game's aerodynamics didn't affect the argument very much.
RemoteTech follows the same pattern. It makes controlling robotic ships obsessively realistic, forcing speed-of-light delay on transmissions and requiring a whole network of satellites to maintain connectivity at all. Some people like the realism and think probes are overpowered without it, while others think it's too convoluted and annoying to bother with. 1.2 added a simpler version of this to the base game, reawakening the arguments.
Life support. The unmodded game still doesn't have it, so most of the arguments revolve around whether it ought to, and if so, what it should be like. The popular mods range from having "snacks" and losing reputation when your kerbals go hungry, to killing them when they are deprived of food, water, oxygen, or electricity for too long. So, if the game ever does include life support, expect immediate backlash over it being too complex and/or too simple.
Germans Love David Hasselhoff: The game is popular everywhere in the world: except in Mexico, i.e. the native country of the developers. In fact, Mexican videogame press barely, if ever, talked about the game and the few Mexican players who played the game thought it was another American or European game. It doesn't help the homepage is in English, without any Spanish-language version of the page and the game itself is in English. In fact, the game avoids including any kind of stuff linking the game with Mexico (except perhaps for the Dia de los Muertos trailer), due possibly to complicated cultural reasonsnote There's also the topic Mexico has no space program to speak about (other than building homemade satellites) and the avoidance of showing any element from the Mexican culture is mostly to avoid Internet Backdraft from both players from countries with advanced space programs and the own Mexican ones.
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: Autopilot add-ons such as Mech Jeb (specifically whether they're helpful learning tools for new players, crutches that take away the fun of learning to pilot manually, a cheating device for players too lazy to learn how to fly on their own, or something that takes away the tedium of flying manually when you've done it so many times already; note that pointing out that it's closer to how real rockets are guided does not help), and Lagrange Points, along with proposals on how to implement them that's already been discussed and dismissed a hundred times.
It's Easy, so It Sucks: A common complaint about career/science mode. Some believe starting off with kerballed missions instead of satellites and probes is unrealistic, i.e. too easy; the "Better Than Starting Manned" mod tries to fix this. Another complaint is that "biome-hopping" lets you unlock the entire tech tree without going farther than the Mun or Minmus. The most popular solution, somewhat confusingly, is to play sandbox, where everything is unlocked from the start (but Self Imposed Challenges may be in effect).
Memetic Badass: Nothing scares Jebediah Kerman, aka "the Thrillmaster!"
Memetic Mutation: MOAR BOOSTERS! has become famous in the geek community, referring to a common "solution" suggested by members of the KSP community when trying to increase the power of a rocket.
Weeks before a large update is released, fans frequently start a hype thread on the KSP forums in which they discuss the update. However, in 0.24 and on from there, there have been "Hype Vehicle" threads in which fans fight over/post image macros about "Hype Vehicles", vehicles that are powered bypure hype. The most argued about vehicles are the iconic HypeTrain and KSP's equivalent, the HypePlane. Here are the 0.24 and 0.25 hype vehicle threads, respectively.
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. I also hope you brought radiators along; when you're that close to the Sun, it's going to get hot.