By the end of Mass Effect 3 you find out that,
1. The Reapers overthrew their creators with the help of the Catylyst to begin the cycle.
2. The Catylyst is nothing more than a rogue AI that was created to solve a problem.
3. That the crucible is needed to defeat The Reapers.
Now if I'm correct I clearly remember hearing that there wasn't going to be an off button or a super secret weapon that would destroy The Reapers. This lead me to believe a few options would be given as how to defeat them.
1. Finding a signal that could shut down their shields and open them up to the ships cannons. This would have been worked on throughout the game however if your numbers were to low you either took very few of your forces to Earth or died in space before reaching the planet.
2. Possible decoy attack by luring all the Reapers to The Citadel. The fleets attempt to destroy the Citadel ( a bluff ) and The Reapers in an attempt to protect it send much of their forces. Shepard and his team then set an asteroid on course for the mass relay and annihilate much of the Reaper forces. They then head to Earth as the last of the Reapers make a last stand there. With option 1 complete you can use a small number of forces to defeat the Reapers in a space battle and reach the planet for a massive war. However without option 1 complete you need a higher number for Military Strength to defeat the Reapers.
3. Cerberus is working on a way to control the Reapers and is in the process of building the device. You can choose to destroy it after attacking the Cerberus base or keep it and fire off the device and amplify it using the Mass Relays to take control of the Reapers and the device works as a computer that can give the Reapers orders. If Shepard is
a. Paragon who saved the Geth / Quarians or both as well as cured the genophage they all rebuild the surviving species homes using the Reapers and this is your super happy ending. They also learn all about the different civilizations and their culture and the galaxy enters a golden age using this knowledge.
b. Renegade he is able to use his status as a war hero to influence many people and uses the Reapers to further humanities power as well as claim many worlds for humanity. If the genophage was cured and Wrex was saved Shepard gets the Krogan several new planets that are taken from other species however peace is maintained. Although if Wreav ( not sure if I spelled that right ) is saved then him and Shepard are able to use the Krogans high birth rate as well as Shepards legendary status and the Reapers under humanities control to bring about a Human and Krogan Rebellion in which they conquer the galaxy. After severely defeating the other races the human and Krogan strip them of their council power and exile them to their home systems. Although if Shepard romanced Tali / Liara / Garrus ( for females )their respective species is allowed to retain a council seat as well as leave their home system. The ruling species run the galaxy with an iron fist and plan to explore other galaxies in the future. Civilization enters a golden age for the ruling species who use the now punished species for labor.
Well those are the ways I had expected Mass Effect 3 to play out. Long story short I also thought the missions on Earth would be longer and have a few more missions and show the horrors of what the Reapers were doing. Although this is nothing like Boiware's Mass Effect 3 I don't really care because if the endings and gameplay were this different from each other the game wouldn't have been out until next year.
So what were you expecting at first?