- "The Bath Item Gift Hypothesis" This is an episode from the second season, and to my mind remains the best episode of the series for any number of reasons. Classic moments include "Look, Leonard, Sheldon's hugging me!", and the repeated refrain of "My leg's killing me, thanks for asking," both of which have become constant touchstones for me. It's the episode that pushes geeky Leonard and bombshell Penny closer to having a romantic relationship after more than a season of teasing it. It's Sheldon's neuroses in classic form. And as far as the cast goes, it seems like a fairly visible moment where everyone, especially Johnny Galecki and Kaley Cuoco, realize just how much they've come to love doing this show, especially the closing moments where Leonard and Penny exchange gifts ("Turns out not letting the bike fall on you is lesson #1!"). If you need any one episode to fall in love with the series, this is it. Oh, this is it. It's a classic for the fans, and easily accessible for new viewers. This is everything charming about The Big Bang Theory, wrapped in a little bow.
- Variety of jokes. Seriously. I read one of those negative reactions recently (source to remain nameless) that took a look and only seemed to notice the dumb jokes, who said they felt betrayed by a series that was supposed to be smart. But the beauty of this sitcom is that it can do smart and dumb with equal aplomb, both with the characters who are supposed to be smart and the characters who are supposed to be dumb. It's always struck me as a little like Frasier, only without all the fussy stuffiness (which I say in a good way, because how else do you describe the Crane brothers?). Sheldon quickly became the face of the series because Jim Parsons so easily realized that his character is not nearly as one-dimensional as he might initially seem. His smartness very easily lends itself to stupidity, because where he's smarter than anyone else in the room he's absolutely clueless socially. But he never stops trying, and never stops assuming that he's always right. And he thinks Leonard is worse than him.
- Character growth. The most shocking thing about the series is its constant character growth, because every single character introduced to date has grown, even though they all started as very specific types. The most shocking would be Howard, who started as the insufferable would-be lothario, but has gone on to develop the most lasting and meaningful romantic relationship, marrying sweety Bernadette. Anyone who saw Howard in the early seasons would never have seen this coming. But it happened. Even Sheldon's always-awkward relationship with Amy Farrah Fowler has evolved. Really! And Raj can finally talk to girls without being drunk!
- Firm understanding of character. This goes hand-in-hand with the variety of jokes, really. Because none of these characters are one-dimensional, they can have any kind of jokes possible, even if it seems Penny is frequently the butt of the same kinds of jokes (for the record she's the only character who not only doesn't have a middle but even a last name, so it only figures; if she ever became successful as an actress that would be the ultimate evolution in the series). The thing is, all of these characters could be said to be in ruts in their lives, and they need each other more than they realize, even as far as Sheldon goes. He might sometimes seem to be unbearable, but, and Stockholm Syndrome aside, these guys really appreciate each other. There's a reason they stick together. They very much complement each other, often in surprising ways. And that's because the writers are always exploring them, examining them from every angle. That's how Howard grew so much.
- Ability to expand the cast. In the beginning there were only Leonard, Sheldon, and Penny, with Howard and Raj at the periphery. And then Howard and Raj grew in importance, and then we got Amy Farrah Fowler, and then Bernadette, and even the emerging Lucy, likely still developing as Raj's intended soul mate. And they all fit so nicely together. They all become a part of the family, and the show never misses a beat. That's incredibly rare. Any other series would have taken the original archetypes for granted. But again, this ties in with character growth. This is a series that looks at all the angles.
- Wil Wheaton. Seriously! Previously known for Stand by Me and Wesley Crusher, Wheaton's cultivated a weird second act both on the Internet and as The Big Bang Theory's best recurring character, Sheldon's natural nemesis in ways you couldn't have imagined until it happened. How someone hasn't tried to build a whole sitcom around him is still a mystery to me, although maybe they have or Wheaton simply doesn't want it. But these appearances alone prove how valuable he is as a comedic figure.
- Love of pop culture. Geeks sometimes claim the geeks on this show get their culture all wrong. But I don't mind identifying as a geek, and think the series nails it just fine. And the ways it happens are always a delight to watch, especially as a Star Trek geek seeing more than just Wil Wheaton show up. Their trips to Stuart's comic book shop ring true, too, although the hapless Stuart is constantly making me rethink the romance of running a comic book shop...Although like everything else about the series he's always hilarious!
Anyway, love the show. Will always be a fan.