This mod introduces some really severe changes to vanilla mechanics. Whether you like change is really up to you, but I felt these were really important to balance industrial modpacks out and to put in place the necessary conditions to start making Thermionics machines.
Not to say "Minecraft should be realistic," because Minecraft gets a lot of its charm from its whimsical nature and its Surgeon-Simulator/Octodad clumsiness. But obsidian has always bugged me the way it's implemented, so let's talk about how magma really works, so we can capture some of those in our design:
Magma forms when heat escaping from the outer core warms the mantle. So when it comes up, it has the chemical composition of the mantle. The mantle has an awful lot of silicon, so when it's melted and refrozen, you can see how igneous (volcanic) rock is usually some kind of glass. How it melts (wet, dry, or infused with gas) and how quickly it cools greatly affect the chemical composition and crystal structure of the rock. We're going to talk about the kind of magma that comes out of vents on the seafloor or volcanoes on the split between tectonic plates. Usually that's a wet melt that's helped along by the fact that escaping lava lowers the pressure inside the vent or flu, which then draws heat up from below. It's kind of a natural blast furnace creating really intense heat that can partially melt the mantle, but often doesn't carry quite as much silicon with it. This is how we get basalt.
Unlike most igneous rocks (like obsidian), basalt is not mostly glass, so it won't create erratic, conchoidal fractures at the slightest hit from a pickaxe. It's also a fairly dense rock, with a specific gravity of about 2.8-3.0 to granite's 2.7 and limestone's 2.5. It really fits the role initially envisioned for obsidian. It's also frequently formed by rapid cooling, which keeps the crystal sizes down and makes a more rocky than glassy material.
Let's face it, the nether isn't diamond tier, and hasn't been for a long time. You can pour a nether portal any time you want, as long as you have a bucket and some lava and water. In modded, block breakers, ex nihilo crucibles, and other innovations destroy any pretense that we were supposed to wait for the high end of the overworld to start exploring, and often the nether is used extremely early-game in its fast-travel capacity to find a free spot in the overworld to put our permanent base. So obsidian, now basalt, is really best expressed as a kind of hard iron-tier rock. With that in mind, it's mineable by iron pickaxes, and its hardness has been brought down to human levels, but its blast resistance and piston-unpushability have been left unchanged.
This was the original reason for tampering with the nether. It ballooned into SO MUCH more, but the biggest problem with the nether is that unlimited free lava is unlimited free energy. So instead, we've developed a new, finite fluid that hurts like death, but makes items float upwards and can't be converted into power. It's also extremely bright and pretty, and can be used to light up your base, but you know, don't spill it, the stuff hurts.
Eventually, we'll have the technology to turn this fluid into new kinds of attractive lamps.
Anyone who has fallen into the void in the overworld knows that the nether isn't really beneath the overworld. It's its own slipspace dimension, and it really wants to be as rich and complex as the overworld, yet have its own creepy ambience.
So we rewired it. It's tall now, impossibly tall. It's got a really complex cave network. Fortresses are still a Thing, but now it has sulfurous vents, stalactites, new dungeons, and, well, just reasons to explore. Remember bubble mountain? We've got bubble mountain. Come to bubble mountain. But maybe don't stay, because it's pretty dangerous.