1- It's about characters who pretty much know they are awful people.
Friends centers on buddies who become a chosen family; The Office is about all the crazy people you deal with at work.
The characters in Seinfeld, by contrast, come off as a bit older and set in their ways — well-aware of how terrible they are. Jerry Seinfeld is playing a version of himself as a midlevel standup comic with a Peter Pan complex; in one episode, he drugged a woman he was dating to play with her vintage toy collection.
His ex-girlfriend Elaine Benes (Julia Louis-Dreyfus) at one point tries to find out from a grieving widower if she can get his rent-controlled apartment. Pal George Costanza (Jason Alexander) inadvertently killed his fiancée by insisting they use cheap wedding invitations which had toxic glue. And hipster eccentric Cosmo Kramer (Michael Richards) caused a riot by accidentally setting a Puerto Rican flag on fire and stomping it out. NBC had to apologize for that episode.
I remember laughing at a lot of those storylines when they originally aired. But it's also some pretty pathological behavior, especially in episodes that don't gel particularly well. Which leads to the next point...
2- It takes a while to find its groove.
Seinfeld actually debuted with an episode called "The Seinfeld Chronicles." Reaction was poor – because the show was awful. Elaine wasn't a part of the episode, Kramer was called Kessler, and Alexander was doing a painfully obvious Woody Allen impersonation through most of his scenes. That episode and the four that followed it were pretty uneven; I'd suggest new viewers start with the second season to avoid the clunkers.
3- It's set in a shockingly white New York City.
Seinfeld also kicked off the trend of setting a popular sitcom in a wildly diverse city, then relegating characters of color to the periphery of every storyline. Nonwhite characters delivered take-out, or were put-upon bosses, clueless attorneys or old guys with a side hustle moving cars from one side of the street to the other to help residents satisfy alternate parking rules.
What they were not, were major characters with ongoing presence throughout the series run. And that was a trend copied by series like Mad About You, Frasier, Caroline in the City, Friends and others. But somehow, at least to me, it looks even worse on Seinfeld.
Add in episodes that have weird notes about race — like the Puerto Rican flag story — and you have a series shouldering a sometimes fitful relationship with characters of color.
4- It's got dated references.
As some others have pointed out on social media, many of the show's convoluted storylines wouldn't even work in a world with cellphones. In the show's finale, Jerry and George give Elaine a hard time for calling a friend to ask about her father's health on a cellphone while walking outside. That seems like a quaint concern now – when some people get offended if you make a voice call at all. Episodes are sprinkled with jokes about old film stars and pop culture, and while some fashions in the show were forward-looking, there's also a lot of mom jeans and big shouldered-jackets floating around.
5- Its real-life stars have had some, um, difficult moments.
Kramer was one of my favorite TV characters, until Michael Richards atomized his career by yelling the n-word repeatedly at Black patrons who were heckling him at a comedy club. Now, it's difficult for me to enjoy his performances, even though Richards has apologized several times and said he was "busted up" by the incident.