I literally just want to know what the big deal was
Various things. People felt that starchild was a curve ball that was completely unexpected and given no notice. In fairness, he was random because we knew nothing of the Leviathans or starchild and we wouldn't learn about either of them until the DLC months later due to starchild refusing to provide any further details.
There was also a lack of explanation in the aftermath of the game and a lack of closure for all of our companions. The Normandy crash lands on a bizarre planet and we are unaware of the fate of the crew.
Then there is the bizarre ending with the child and grandfather talking about "The Shepard" and how apparently Shepard's actions became legend. Not to mention, the three options at the end of the game were incredibly vague and ambiguous with some choices contradicting what you did previously in the game. The entire ending of ME3 was just rushed, incomprehensible, and poorly executed when it first released.
Leviathan, Citadel, and Extended Cut helped to rectify a lot of issues. However, even the last ten minutes of the game on the Citadel after being hit by Harbinger with all the strangeness that was happening threw many folks off and was disappointing as it came across as if BioWare was trying too hard to be clever and thought-provoking. Some also complain about the forced sacrifice/death of Shepard, although it's debatable that Shepard survives in High EMS Destroy with the breath sequence.
Ultimately, it came down to a matter of the ending was not what many expected and thus was a disappointment.