Reading Snowravens thread (http://www.mmo-champion.com/threads/...+are+the+titan), and I really liked the idea of the Titans just being a really advanced race, and as I thought more about it, a lot of loose ends in lore seemed to come together. This is why here I am speculating on the Origins of the Titans, why Sargeras really fell, and why the Titans have 'fallen'
*Everything below is speculation, don't take it as fact*
The Titans were once a mortal race. However, they were more technologically advanced than any other race in the universe. They thus became the first race to perfect planetary travel. Like any mortal race, they were curious and went about discovering the universe. Before this, the Titans knew nothing of Magic.
Than they found it. A dimension in the universe where, for some unknown reason, both there machines and amazingly they themselves became stronger, faster, and all together better. They were perplexed over this, and began to study this source of power, naming it Arcane, and naming the dimension they found it as the Twisting Nether. So, they studied an manipulated Arcane, achieving great success as they did so. But as time went on, they realized something. Though there will power was enough not to succumb to the addictive properties of the Arcane, there fleshy, mortal bodies would eventually give out. So, the Titans made themselves into machines of pure, unimaginable power. Realizing the corrupting nature of the Arcane, they set out across the universe, to make sure no other race would be as corrupted as they almost became.
They made creations (ex. Doomguards), that would go around detecting Arcane magic, making sure it wasn't used, and if it was, it was only used in the smallest of capacities. Obviously, certain races like the Eredar would go undiscovered. They did not want the other mortal races to become corrupt, but they were too late. They came upon races like the Dreadlords, and realized that Arcane is so addictive and corrupting, that after it corrupts the hosts body, it corrupts itself. They named this corrupted magic Fel, and these corrupted mortals as Demons. Thus, they made an army that would go around the universe, led by Sargeras, imprisoning and destroying these Demons. The rest of the Titans went about there business of bringing order to the Universe.
They then came to Azeroth. The Titans wanted to test something out; Instead of blocking mortal races from Arcane magic, they wanted to see what would happen if instead, they had mortal races grow alongside Arcane. So they implemented the Well of Eternity, a lake connected directly to the Twisting Nether, bristling with power. There stone and mechanical races were purposefully designed to eventually become fleshy, and have the ability to reproduce. Afterwards, the Titans set off into the Great Dark, but left more fail safes in Azeroth than they had any other planet. But the Titans weren't able to study our progress, because they had fallen.
While they were bringing order to the universe, the most unimaginable thing possible happened. Sargeras, the one leading the charge in the Twisting Nether, became corrupted. The Titans had made themselves machines so there bodies couldn't fall to corruption, but Sargeras was a different story. He was exposed to Arcane and Fel for so long that it began to taint his mind. The Titans, despite seeing themselves as perfect, realized they could still fall victim to mental corruption. Sargeras' body warped and became similar to the demons that he faced. The Titans then remade the army Sargeras lead, putting Aggramar in charge, and changing their goal to that of stopping the Burning Legion.
But for the most part, the Pantheon has fallen silent. They are worried they may to eventually fall victim to the corruption, and have been very hesitant to face Sargeras for this very reason. This is why Azeroth is so important to them. If Azeroth succeeds, than the Titans no longer have to continue there work. Maybe they want the mortal races of Azeroth to replace them in their universal goal? I dunno, its all a mystery.