In the beginning there was BuildCraft, and IndustrialCraft2, and the players had pumps and generators and lo, it was good.
Then the Nether tempted some. The vast lakes of lava were pumped and fed into the hungry maws of geothermal generators. For a time all was well, until more than one player decided to pump the Nether for power. The flowing lava and the many flow recalculations per second caused the server to flail and gnash its teeth, and there was much wailing and cries of "Why is the server so #&*$ing laggy? This server sucks!" The server ops wept for their children even as they banned pumping lava in the Nether, knowing they had no choice, and there was much sadness.
The modders, in their wisdom, thought long about this tragedy. The players love the geothermal generators, they said amongst themselves, but it causes much distress to the servers and the players cannot use their beloved pumps. What to do? So Thermal Expansion added the Magma Crucible so that there would be a source of lava without needing pumps and the flow blocks the pumps begat.
All was well for a time. But players still, in their newbness, pumped the Nether and had to be explained the uses and virtues of the Magma Crucible. And players, in their leetness, discovered that the Magma Crucible could generate free energy in the right configurations, and there were those that lamented the loss of the beloved Nether pumping stations and looked upon the vast burning lakes with longing, and some fell to temptation and were banned. The forums overflowed with cries for this injustice to be fixed and the Nether to be returned to its natural place as a source of geothermal power.
Greg heard the cries of the fallen, and held them to his bosom and gave them succor. "Here, give me your IC2 pumps and your BC pumps (but speak not to me of the RedPower pumps, for they are an abomination unto me). Give me your pumps, and take from me the Advanced Pump. Take this, my gift, and pump the Nether! It will pump more slowly, but while its pace be tempered, it will drain the vast lakes of lava without creating flow blocks, and lo, it will be no laggier than a quarry for it will merely remove blocks and cause no recalculations! Rejoice, for I am thy salvation!"
And the players beheld the Advanced Pump, and the scales fell from the eyes of some and they saw it was good. Those who knew of the old wailing and gnashing of teeth, and who followed the mystic movements of the mod developers saw what Greg had done and saw the wisdom. They could pump the Nether on shared servers, or run multiple pumps in their private games, and they would no longer see only a slideshow. There were players though who were but babes in arms when Nether pumping was learned to be nefarious and evil, and they looked upon the gift of the Advanced Pump and in their arrogance spat upon the ground and bit their thumbs in contempt.
Thus was confusion and disarray born. Greg bowed his head in sadness, but took comfort in the joy that he brought to the enlightened few. He knew this was the way of things, as he was already no stranger to the jibes of his detractors. He smiled his secret smile and turned his eyes from the confused and angry players, and basked in the love of those who understood him.
And so it was, and so it shall be again, for time is but an ever-turning wheel.
This is but a story, but in it there is the seeds of truth. Remember it and tell it to your children and your children's children, so that our people will remember and always walk in the light.
Best Answer
Hexxit adds three pages of achievements to what vanilla Minecraft has, full of adventuring and excitement. That gives you a kind of a map of its objectives.