Makes your world more reactive.
I see Minecraft as a static game. Nothing happens without your intervention. This isn't bad, but sometimes I want to feel as if I am a part of the world and not the other way around. These mods make the world react and retaliate to your actions. If you aren't careful while mining you might cause a cave-in, and while you are being crushed by blocks your crops will still grow. You might notice some ore on the ceiling of the cave, but what you didn't notice was the dripping water. Oops... now you're stuck drowning in a pitch black cave you accidentally flooded. Meanwhile over at your base, your puppy grew up. Everything you do has a reaction, borderline retaliation, from the world.
The amazing mod Simple Clouds by nonamecrackers2 adds local weather. This has the cool side effect of making scenarios where you can hear a thunderstorm rolling in.
The combination of Simple Block Physics, Flowing Fluids and True Darkness makes cave exploration terrifying. The risk of causing a cave-in plus flood leaving you in complete darkness adds a nice thrill to cave exploration.
Building is way more of a challenge. You actually have to think about structural integrity and the order you build things. Can't have a roof without supporting walls.

This modpack contains mods that revamp Minecraft's fire, water, block physics and weather.