Although Sheldon became more prominent as seasons went by, he was a co-lead in season 1. Leonard was the show's de facto protagonist. As such, Sheldon was a much more robotic and awkward character, not as lovable as he would become in later seasons.
Season 1 Sheldon was more stoic, less involved in his social relationships, and all-around more "weird." His character arc turns him into someone more in touch with his feelings thanks to his friendships and relationship with Amy, making his season 1 version almost unrecognizable by the end.
In season 1, Howard is a creep. He is rude, invasive, insensitive, and borderline harassing of nearly every female character. He behaves appallingly to Penny, giving her off-hand compliments that make her uncomfortable at first and eventually angry.
Later seasons of The Big Bang Theory show Howard's considerable growth, thanks to his marriage to Bernadette and becoming a father. Gone is the horrible and reprehensible character from season 1, and thank god for that. In hindsight, it makes no sense that someone like Penny would endure Howard's horrible ways.
Leonard is The Big Bang Theory's de facto protagonist in season 1. The show introduces him as a socially awkward but highly intelligent experimental physicist who develops an instant crush on his new neighbor, Penny. Leonard spends most of the season developing his relationship with her, culminating on their first date.
While clumsy and inexperienced, Leonard in season 1 is far from pathetic. However, later seasons turn him into an overly needy and insecure character who emotionally manipulates Penny. A toxic personality is certainly one of the things that aged worst about Leonard's character. A shame because Leonard is a pretty cool guy in season 1, but his charm wears off somewhere around season 6.
The Big Bang Theory introduces Penny as a struggling actress who works as a waitress to make ends meet. She spends half of the show suffering from dire financial distress, partly because her acting career is going nowhere and partly because of her compulsive need to buy clothes.
Penny later gets a high-paying job as a sales representative for Bernadette's company, effectively abandoning her dreams of acting stardom. Thus, it's strange to rewatch season 1 and see Penny going to failed auditions and working at The Cheesecake Factory; it almost seems like a completely different character!
Raj is The Big Bang Theory's resident third wheel. He spends most of the show alone, mainly thanks to his social awkwardness and overly needy personality. Throughout the show's first half, Raj suffers from selective mutism, which prevents him from talking to women unless he drinks alcohol.
However, he overcomes his issue in season 6, becoming brattier and slightly more annoying. Season 1 sees Raj spending most of his scenes silent, whispering in Howard's ear or lurking in the background. Compared to the livelier, chatty version in later seasons, the mute version of Raj in season 1 is nearly unrecognizable.
Season 1 of The Big Bang Theory introduces the guys are very close-knit because of their difficulty with social interactions. They spend almost all their time together, playing games or pursuing hobbies other people would consider "nerdy" or "uncool."
Future seasons shift their dynamic considerably, especially after Amy and Bernadette's arrival. The guys become less dependent on each other and begin pursuing other interests outside their friendship. Whereas season 1 sees them attached by the hip, later seasons explore their interactions with other characters, including Stuart, Zack, and several other supporting characters.
Initially, The Big Bang Theory's main hook was its mix of science and humor. The show's tagline was "Smart is the new sexy," implying that Penny would eventually fall for Leonard because of his brains. Indeed, much of the show's comedy revolved around the guys' elaborate speeches about physics concepts, contrasted with Penny's lack of understanding.
This angle became less important in future seasons as storylines began revolving around relationships, marriages, parenthood, and such. It made sense for the show to grow up alongside its characters, but the change compared to season 1 is notorious.
For all its strengths, The Big Bang Theory's take on Penny in season 1 was weak at best. The show treated her as a stereotypical dumb blonde, carefree and unintelligent, whose bubbly behavior contrasted with the guys' more intellectual and geeky approach. It's no wonder many of Penny's cringiest moments in The Big Bang Theory happen in season 1.
The show developed Penny's character in later seasons, giving her more depth and agency. This makes rewatching season 1 more awkward because Penny is at her most one-dimensional during the first season. Her role is only to serve as a pretty contrast to the guys' nerdy dynamic, which is a disservice to the character and Kaley Cuoco.
The Big Bang Theory had several dunny side characters, but Leslie Winkle was one of the first. She is a co-worker at Caltech and an occasional love interest to Leonard. Leslie has a no-nonsense attitude, viewing human relationships as a transaction and removing feelings from the equation.
Leslie plays a prominent role in season 1, forming a love triangle of sorts with Leonard and Penny. She even becomes a regular cast member in season 2, thanks to her popularity and the spice she brings to the plot. However, she disappears less than halfway into the show, and by season 12, she is but a distant memory.
Howard's creepy behavior involves every woman in his life, most prominently his overbearing mother. Although she is never seen, Mrs. Wolowitz plays a huge role in the plot throughout the show's first half. She is a mother figure to everyone in the show, even if the guys are usually disturbed by her physical appearance and possessive ways.
Mrs. Wolowitz dies in season 8, and the show makes few mentions of her in the last four seasons. It's strange that such a prominent figure became so distant by the show's end, to the point where rewatching season 1 feels strange, considering how much Mrs. Wolowitz is in it.