About - aka What is Reasonable Realism?
    Several years ago, heck before I even got into modding Minecraft, I was jotting down notes, lots of notes, about what I'd seen other mods do: things I liked, things I didn't like, things they didn't do but that I'd like to see. Stuff like that. Most of these notes were heavily inspired by TerraFirmaCraft and IndustrialCraft, with a little from BetterThanWolves, although BTW fell more into the "what I didn't like" camp, but knowing what doesn't work for me helped me work out the design and understand what I did like and why.
    But my one guiding principle was "I want realism, but not realism for the sake of being realistic. I want it to be reasonable, both from a gameplay standpoint and from a technical standpoint (can I program this?)." A couple of features ended up being harder to work out, either from a gameplay perspective or a technical perspective, than I'd originally anticipated, but it's those challenges that make me a better programmer.

This one's easy to describe. It won't necessarily be easy to play. I wanted ores like in TerraFirmaCraft or with COG, but I also knew I wanted them to be precisely anything but. The first problem was that ore was too easy to get in quantity. The second was that it was damn near impossible to find. Harder Ores fixes both. Happy prospecting!

This mod is extra extra small, it does 1 thing:
It grows unique flowers near concentrations of ores, both during worldgen and when using bonemeal. This feature was extracted from Hard Ores, allowing the mechanic to be used on its own.

Ever thought to yourself, "Gosh, farming wheat is really easy. After the first day or so I hardly notice my hunger bar except when I forgot to bring some/enough food with me. Quick, pop back up to the nearest resupply chest!" Well, with Harder Wildlife, that is no longer a concern! (A bigger concern? Damnit English!)
Harder Wildlife slows down crop growth rates by an approximate factor of 10. "Best case growth" of 37 minutes? Pssh, try 6 hours! To compensate, animals drop more meat, but they also age, die, and reproduce. It also causes trees to eventually die and replant, as well as add a seasonal cycle (which also affects crop growth). Snow in the plains? Rain in the desert? Absolutely!
Notes: The tree thing is entirely optional and is not perfect. It should function relatively well in most cases, but if you find any oddities, you may be best off just disabling it.
Seasons can cause minor graphical discrepancies in grass and leaf color along chunk boundaries, as the alterations do not cause a render update. This is normal.

Harder Farming is an alteration to the Minecraft farming mechanics. Animals automatically repopulate and die off, crop growth is slowed and takes into account biome attributes.  It takes more wheat (seeds, etc) to make animals go into "love mode," but they drop more meat and leather so it is no longer painfully scarce. Killing a sheep gives more wool than sheering.
This is a rebuild of Harder Wildlife for 1.10.2, minus the seasons, added mobs, and tree die-off as those additions did not fully work as desired or were not terribly popular (monitor lizards).  This also allowed the removal the dependency on COG.

Harder Underground attempts to make the underworld--that is, anything under that thin layer of dirt you call the surface--significantly less boring, homogenous, and oh yes.
Safe.
Harder Underground makes you work for your precious resources. You will face toxic, unbreathable, gasses and a ceiling that just won't stay up there where it belongs. Try not to get buried alive.

This one stands on its own and is a really small mod, but it has the no-nonsense stuff that vanilla lacks to really automate any kind of resource production. Most other mods bypass the vanilla methods (cart and hopper) this one encourages the cart and hopper. More info on the wiki as really every block needs its own explanation.
If used with HardLib, Comparators can examine Item Frames, comparable to the feature introduced in Vanilla 1.8

There is a wiki. It's practically already completely documented, but...you know, its a wiki. Its always possible I forgot something.

Other mods in this genre
There's a few other great mods out there that fit the idea of making the game more realistic, and in that way that makes the game more fun. I'm always interested in finding others, but here are **s I know about:

• Walkable Slopes - Collision volume of dirt blocks converted to stairs/slabs as the surrounding terrain allows, letting the player walk up gentle hills and slopes without jumping.
• Flowing Rivers - Realistic rivers with flow, width, water falls, and direction.
• Growth Craft - Another take on farming. Should be compatible with Harder Wildlife.
• Air Overhaul - Instant death when you run out of air. Makes respiration enchantments more viable (may be too harsh).
• Better Rain - Rather than on/off, rain can come as a light drizzle to a torrential downpour.
• Biota - Adds nutrient tracking for plants, causing plant growth and death based on nutrients (tracked at the chunk level).
• Somnia - Simulates the world, even while sleeping.
• WTFCrops - Likely not compatible with Harder Wildlife, but it's WhiskeyTangoFox's take on altering the vanilla agriculture.
• Realistic Torches - Torches burn out. Use matches (wood slabs + paper, although I'd have used sticks) or flint and steel to relight them.
• The Seasons Mod - ACTUAL honest to god seasons. Uses PML, making it harder to install, but it's worth it for the fall colors. Probably not compatible with Harder Wildlife.
  

These aren't my cup of tea, but they follow similar goals:

• Hunger Overhaul - Making hunger and food matter more.
• Enviromine - Thirst, air quality, food spoilage, gasses.
• The Vegan Option - Adds alternative recipes/items allowing players to never kill any animal or hostile mob and still have full access to the game.