One of the biggest reveals was that Sheldon had a best friend named Tam, an outsider like him with whom he ate lunch every day in the library and even hung out with after school on occasion. The Big Bang Theory recognized this discrepancy by introducing an adult Tam in an episode in the final season.
The fact that Tam was never mentioned was explained away by noting that he moved after high school and got married, which upset Sheldon because he felt abandoned. Nonetheless, Sheldon wasn't completely friendless and alone in high school like he described. He also never mentioned Paige, it's worth noting, another person of his age with whom he had a cordial relationship.
One of the biggest inconsistencies in Young Sheldon compared to The Big Bang Theory was that George was not nearly as bad as Sheldon described him to be. It could have been the fact that Sheldon saw things through a child's eyes as being worse than they were, or that Sheldon convinced himself his father was worse to help deal with his death.
Nonetheless, George was a devoted family man and often went out of his way for his kids, even if reluctantly so. While he enjoyed drinking beer at home and frequented the local bar, he did not come across as someone who had a serious problem with alcohol and never appeared drunk in front of the children. It could be that Sheldon simply equated drinking with intoxication and assumed his father was absent and neglectful in an embellished view of this.
While Sheldon and Missy didn't see eye to eye on many things, they were actually closer than he might have implied. While they grew farther apart as they get older, especially with him living on campus and her back at home, as younger kids, they fought but also were there for one another when needed.
It was Missy, in fact, who helped Sheldon get over his nerves about going to college and giving his valedictorian speech at high school graduation. There were many times when the two came together to offer love and support for one another. Seeing how much they loved one another has fans feeling sad that the twins were not closer in adulthood.
Sheldon spoke highly of his Meemaw and fans got to see a bit of their closeness in the episode when an older Connie came to visit. He also clearly had a loving relationship with his mother, as evidenced whenever she visited. But Young Sheldon shows just how close he and his mother were.
Mary fawned over Sheldon, worried about him all the time, and often gave him more attention than her other children. He also clearly respected his mother even if he didn't agree with her devout religious beliefs. So much so that when Mary was, at one point, questioning her faith, it was Sheldon who helped her get it back.
Even more shocking than the fact that Sheldon had a best friend in high school was that he had a group of friends he hung out with in college. It's one of the plot holes in Young Sheldon fans can't overlook. Even Paige was flabbergasted to learn this when she went to visit him, especially given how she was having such a hard time relating to her much older peers.
They all lived on the same floor in the dorms and shared a mutual interest in video games and Dungeons and Dragons. Thus, they didn't care much about Sheldon's age or his idiosyncrasies. He was another player, and they were happy to welcome him to the games. Sheldon even felt a twinge of peer pressure when he went overboard with soda and candy and his mother had to save him from the terrible stomachache after the fact.
Sheldon never had anything good to say about his brother, referring to him as a goof, untelligent, and aimless. But Georgie was clearly a good person, even if he was misguided. Even though he wasn't interested in academics, he worked hard as a teenager at various jobs, and when he found out Mandy was pregnant, he tried to step up any way he could. In fact, he tried desperately to be there for her despite her not wanting to have anything to do with him.
Yes, he often made bad decisions, did not excel in school, and wasn't particularly smart. But given how successful he became later in life, Georgie obviously learned some things about business along the way and used his street smarts to excel financially.
While fans didn't like the way the show tried to justify George's eventual cheating by suggesting Mary had an affection for Pastor Rob, it was indeed true, even if she refused to admit it to herself. George could seemingly sense it as well. It's likely Mary – one of the most likable characters on Young Sheldon - never physically cheated but she did have impure thoughts (and dreams!) about her once co-worker.
While this doesn't excuse what fans know George eventually does, it shows that Sheldon might have viewed his mother with blinders on. He might never have known that she felt an emotional connection and sexual attraction to someone else. Nonetheless, Young Sheldon revealed that to be the truth.
In one episode where Sheldon and his father bonded, they go to CalTech to attend a lecture by Stephen Hawking. They even go into the iconic cafeteria where adult Sheldon and his friends and co-workers are often seen having lunch on The Big Bang Theory. In one scene, the younger Sheldon even mentions he could see himself attending that school one day.
It was a sweet callback to the original but also showed that when Sheldon began working at CalTech, it wasn't the first time he had stepped through those doors. As someone with an eidetic memory, it's possible Sheldon might have secretly felt memories of his father every time he stepped foot in the door.
Oddly, Dr. Sturgis was never mentioned in The Big Bang Theory, despite having such a crucial role in Sheldon's life for many years. At first his professor, he later became a close friend and confidante, more so during the time when he was dating Connie.
Dr. Sturgis' mental breakdown – one of the times Young Sheldon dealt with serious issues - was rough on Sheldon. Fans are yet to find out what comes of Dr. Sturgis, but the fact that Sheldon had such a close friend with whom he could banter intellectually when he was younger was news to fans. Given how Sheldon completely shut out his memory of Tam, it could also be explained that Sheldon blocked out any memory of Dr. Sturgis as well because it was too painful.
Sheldon didn't talk much about the inspiration for his love of science and physics, but in Young Sheldon, it's mentioned on numerous occasions that his grandfather shared a similar love and passion for STEM fields and instilled this in Sheldon.
While Sheldon was clearly already-intelligent from a young age, the relationship he had with his grandfather helped fuel his interest in the subject and might have been a motivation for pursuing it further. The adult Sheldon talked a lot about his parents, siblings, and grandmother, but losing his grandfather at such a young age was something he only ever mentioned on occasion. And he never talked about how his grandfather was a science buff as well.