All the lists of the ending variations talk about "good control/destroy", "bad control/destroy", but there is never an explanation what this is based on. In the good case, you can see soldiers cheering, in the bad variant, they don't. So what does determine whether by choice results in a 'good' or 'bad' ending. Specifically, what is the reason the soldiers would cheer, or not cheer?
Mass-effect – Mass Effect 3: “good” vs “bad” ending variations
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Best Answer
TL;DR If everyone you know is dead, no cheering. If they lived, yay ensues.
Image taken from this video.
HERE BE SPOILERS!
If Shepard chose to destroy the Reapers, a big wave of flame sweeps over the Earth. If you have a low effective military strength (EMS) score, the cut scene will show the flame wave destroying everything in it's path, including Reapers, buildings, and humans. Earth is destroyed.
If you choose to destroy the Reapers with a medium EMS then the humans live, however Earth is heavily damaged by the blasts and buildings get vaporized.
If you have a higher EMS, the energy wave destroys the Reapers but leaves buildings intact, and does not kill the humans. Earth is not destroyed.
If Shepard chose to control the Reapers, the cutscene will show some electric activity on the Reapers, and the Reapers will leave Earth. With a low EMS, many buildings are destroyed in the process. With high EMS, buildings remain intact.
If Shepard chose the Synergy path, the Reapers depart from Earth and Earth is saved.
In cases where humans survived, the soldiers on the ground cheer as the Reapers either leave Earth or are destroyed.
To find out exactly what levels are needed, check out this answer.