You know how some people relapse from addiction and just go crazy, followed by an overdose? I went like that but with KSP mods. i have a version running the following mods, mostly playing well with each other:
Ferram Aerospace Research (more realistic aerodynamics)
Deadly Reentry (makes stuff burn up)
TAC life support (self explanatory)
VOID (heads up display kind of thing)
Kerbal Engineer (gives data about stuff, in flight similar to VOID)
AIES Aerospace (bunch of more rockets and stuff)
DMagic Orbital Science (science stuff for probes and landers)
Engine Ignitor (more realistic rocket stuff, limited number of starts, some more stuff)
Enhanced NavBall (self explanitory)
FireSpitter (not sure, it was required for other mods to work)
Kethane (mining resources on planets)
KSPX (more parts again)
KW Rocketry (more parts)
Infernal Robotics (Moving parts like hinges)
Kerbal Attachment System (guys can build some stuff in space, carry parts)
Docking port alignment indicator (don't use this much, haven't figured out what stuff means)
Near Future everythihng (multiple mods, adds stuff after the end of the tech tree, more tech)
Final Frontier (adds medals and awards to kerbals who do stuff)
NovaPunch (Parts)
Procedural Fairings (adds fairings)
RealChute (realistic parachutes)
RLA everything (3 mods, adds parts)
Station Science (adds science stuff for space stations, makes them more than just fuel stations)
Interstellar (adds lots of futuristic parts at end of tech tree like "Near Future")
Kerbal Alarm Clock (makes keeping track of multiple missions easier, haven't messed with it much yet)
Active Texture Management (reduces memory usage, allowing KSP to not implode)
There are some other mods required that come with the ones above. There are a few mods that are more complicated, though, like:
Real Fuels w/ stockalike engine pack. Instead of "liquid fuel" and "Oxidizer", it uses lots of different kinds of fuels used by real rockets. Some engines have different fuel configs, too, each fueld with it's own characteristics. The mod even supports other mod engines, so AIES rockets will work with this mod. What it doesn't support are new kinds of fuels added by other mods, like the Argon gas required for certain rockets with the Near Future pack. Argon tanks come with the Near Future stuff, so it isn't actually a problem in game.
It also adjusts weight, since most stock parts are WAY too heavy compared to real life. This is because Kerbin is tiny compared to earth, and it makes things more challenging. You can set it to give you normal heavier parts, though, if you want.
The realistic weight parts work best with the "Real Solar System" mod, which as you might expect replaces the solar system with real sized/spaced planets. That makes everything wicked hard. Like, you need double the DeltaV to get into orbit, which is hard with stock rockets. Real Fuels makes this easier with the realistic weight parts, plus there are other mods to scale things up (like Realism Overhaul) but I couldnt' get those to work without massive glitches.
Also some of the mods needed a few file 'tweaks' to get them to work.
The new parts surprisingly don't make the game crazy easy, most are well balanced and you don't get the awesome stuff until after the original tech tree, so they can take thousands of science points to unlock. The insanely high efficiency engines usually also need crazy amounts of electricity, which means nuclear reactors, which are heavy and generate lots of heat, etc... It doesn't get easier, just awesome-r. The aerodynamics one, the engine ignitor, life support, and reentry one make it a LOT harder, actually.