Now that I have the basics of Kerbal Space Program mostly mastered (I can get to orbit easily), it's time to push the difficulty up a notch by installing DRE (Deadly Reentry), FAR (Ferram Aerospace Research) and TAC Life Support mods.

DRE requires that your reentry craft have heat shields, parachutes can't be deployed until below 350 m/s.  That will result in lots of toasty kerbals if you are not careful.

FAR boosts the difficulty of getting out of orbit because it introduces a better aerodynamic model over the simplistic KSP model.  That means your rockets can be torn asunder during lift by air forces flowing over their surfaces unless you use fairings and other aerodynamic covers.

TAC boosts the difficulty of keeping your Kerbals alive for that "one more orbit" that is needed to get them back on the ground.  You can no longer leave Jebadiah in orbit for months and years at a time without food and water.