Playing RO+RSS only, for year or two. Early game is fun, LO stuff and going to Moon, and back, is fun. But after Moon, IRL time and effort needed to plan, build and execute missions to other planets are considerable larger, to do orbital randezvous to build larger spaceships is pain for me, to do space station logistic even larger. To collect more science for better ship parts to collect more science is kinda, well, lacking higher goal? Only once start building something that should be LO space station.
What is making everything a pain for me is that at one want point VAB exe stop working and/or bugs kill it, not fun to for hours fight game itself to make your 1000+ tons rocket work as it should at all, pass launching first step. So usually around Moon missions and kerbals on it, and back, I stop current game and wait for next bunch of updates and then repeat the cycle. Missions have some load game but VAB, I hate wasting IRL time in VAB from bugs.
Atm I should do multiplying launching boosters made from engines, fuel tanks and parachutes, 100 tons and more, nothing fancy, and Im trying NOT to do them as long I can as I know what pain it will be to simply try to make that work and dont kill the exe in VAB.
EDIT: KSP vanilla is sooooooo easy, so little delta v needed to move around that is to easy and then boring, you can send rockets with starting first couple tech one way to Duna easy. Dont anybody educate them self with Scot Manley videos So I cant really understand people who cant go interplanetary in vanilla O_o