The most important thing I think I learned playing KSP was that I wanted to be Wernher von Kerman, a passion I'm still pursuing through college.
That said, there's a list of gameplay related things I learned that are really important (to me)
1. Do it yourself. There's a lot of importance to that, the sense of accomplishment the first time you orbit, the first time you dock, the first time you land on the Mun, it's truly incredible.
2. That said, MechJeb is a beautiful thing. For planning interplanetary transfers or maximizing efficiency MechJeb is indispensable for me. Kerbal isn't about the dozen launches of the same booster with slightly different payloads to build an interplanetary vessel, it's about the vessel, and the wonderment when you take that monstrosity to Duna and beyond. That's where MechJeb comes in for me.
3. The community for KSP might be the best gaming community I've ever had the pleasure to immerse myself in. The range of tutorials, spacecraft, and stupid inventions never ceases to amaze me. The best part, though, is the modders. Kerbal wouldn't be the same without its mods. The base game is incredible (worth the 20 dollars I sunk into it a million times over) but the addition of everything from life support to future stuff to 7.5m parts to entirely new solar systems and realism overhauls make it something truly special.
4. The little things count:
a. Check your staging. You will blow yourself up on launch. (or worse, hours later)
b. Remember your Re-Entry kit (retro-boosters, heat shields, parachutes, etc.). Every. Time. I've killed Jebediah enough times to learn this lesson.
c. Electricity is always important even when you don't think it will be. You will not have enough, ever.
5. Have fun! KSP is only as hard as you make it, so blow some stuff up on the launchpad.
