The two biggest arcs in Mass Effect 3 are the Tuchanka arc and the Rannoch arc. One was about curing the krogan of the genophage and helping restore their future; the other was about fighting for the quarian homeworld (which they had been kicked out of 3 centuries ago by the geth). You [can] accomplish both these things in Mass Effect 3, achieving the hopes and dreams of your friends Wrex and Tali.
Now, all this depends on the mass relays. Without the mass relays, it takes centuries to cross the galaxy. Tali and Wrex join you in the fight on Earth to get to the Citadel. In order for them, the krogan armies, and the quarian fleet to get back to Tuchanka and Rannoch, they need those mass relays.
Then the ending happened. The relays were destroyed no matter what, with no indication that they could be repaired (building mass relays is far above the various races' technology level). All those fleets you brought with you are now essentially stranded in the Sol System. Wrex, Tali, and all those krogan and quarians you brought with you will never see those things you fought hard for them. Not to mention the various asari, salarians, turians, etc., who are stranded with you. Earth appears devastated, so the only future in store for them is a brutal war over the last resources of Earth. One that won't involve the turians and quarians, because they can't even eat the food that grows on Earth. They'll starve quickly while everyone else fights for what scraps are left on Earth.
The original ending was pants-on-head stupid when you think about the logical conclusion from all those events. From a story perspective, that was the thing I was most pissed off about. The soldiers in Sword Fleet literally were no better off than if the Reapers had killed them. And that's why I am glad that the Extended Cut changed the ending making the mass relays destroyed to only "heavily damaged", which implies that they can be repaired, people will get out of the Sol System and see their homeworlds again, and not kill each other over food.