I have just experienced the abomination that was the Mass Effect 3 ending. I will now write a better one in less than 20 minutes.
Everything plays the same up til you are running toward the beam. The camera pans back and you see the entirety of your team mixed with various species' best forces storming the beam. Depending on your galactic readiness, various teammates fall dead and the rest are merely falling behind.
Anderson, also in the chase, announces that the beam is beginning to retract. "They know we are close!"
You, Shepard are the only one to make it into the thinning beam of light.
Aboard the Citadel, you meet the Illusive Man at the control panel. You then debate with him. He explains that the two of you can rule the Galaxy, as the heads of humanity, by controlling the Reapers. "The power is at our fingertips Shepard!" He goes on to explain that the Crucible/Citadel combo allows you to reprogram the Reapers permanently but it also is able to remotely disable all reaper tech, the Mass Relays included. "We will loose everything!" The Illusive man claims.
You then can paragon/renegade him into stepping away from the control panel. Paragon he shoots himself in shame. Renegade you shoot him clean. The third option on the right of the decision wheel allows you to agree with him.
You then decide the fate of the galaxy. Take control of the reapers and enforce your will, or you destroy them.
If you choose to control the Renegade option of controlling them, a cut scene unfolds showing a signal bounce across all Mass Relays, and showing Reapers across the Galaxy shutting down and becoming lifeless, then slowly awakening and ascending to space to the cheers of civilizations everywhere.
If you choose to destroy them, the same cuteness plays except they don't ascend to space and lay there lifeless. The Mass Relays also go dim after the signal is broadcast. The same cheers are seen across the Galaxy.
Screen fades...
Now aboard the Citadel, the repairs are under way with various species seen helping eachother out. You sit in the human embassy next to Anderson and James Vega enters the room. "They are ready for us, Commander."
Anderson looks at you "Better not be late to your own award ceremony."
You enter the Citadel Tower where cheers are loud from the many species present. You are presented with some award by the council, now with Anderson reassuming his post. They announce that due to the allegiance you command among the races, you have been given the authority to pick and appoint one additional counsel race/representative.
You make your decision.
After the counsel dismisses, you are reminded to attend the celebration in the presidium commons.
You then take control of Shepard at the party and make your way around the celebration speaking to all your surviving squad and other people of importance. Through these conversations your actions are referenced, such as if you cured the genophage or not, if the Geth were spared etc. Depending on those decisions, it is revealed that the Krogan are being brought to a new planet, much resembling their homeworld before the nuclear wars. The Geth are still helping the Quarians settle their homeworld. EDI informs you that work is already underway by the best engineers in the galaxy to restore the Mass Relays, and while they are still down, communication between the words remains intact. The Citadel remains the center for galactic politics and Earth is now populated by a large number of all the species who took part in the united fight, seeing as they are for the foreseeable future stranded on Earth and the Citadel.
You have touching conversations with all your teammates in attendance and are also allowed to reveal your future to them. You are able to decide to return to Earth, or take up a political position of influence on the Citadel. You are able to invite your love interest to accompany you wherever this decision takes you.
You then make your way to the exit of the Commons and the camera takes control. It pans across showing Joker dancing with EDI, Tali drinking through her emergency delivery system (straw), and the rest of your squad doing their appropriately touching thing.
Credits roll and a post credit scene is of you with your love interest (if they survived) on Earth or at home on the Citadel. Witty banter ensues.
DLC #1:
You are back on the Normandy six weeks later. Joker informs you that he hopes your time on Earth/Citadel was relaxing as the Mass Relays are up and running and you are needed. Admiral Hacket consults you from the vidcom informing you of the state of the Galaxy. A rogue Krogan general is plotting an attack against whoever and there are other problems facing the races. Yada yada yada...