Here we have a minor disagreement over definition of terms. I’m using the much narrower definition of “sitcom,” as defined by folks like Norman Lear and Harlan Ellison, where the “situation” part comes from the initial situation established by the show’s premise, as opposed to the broader (and arguably more common) definition, in which the “situation” part refers to the episodic definitions. The broader definition really only serves to distinguish situation comedy from stand-up or sketch comedy. For taxonomic purposes, a better term for this type of show is “episodic comedy.”
These shows are much more unconventional (and IMHO better) than Seinfeld – no argument there. However, using Lear’s definition, The Young Ones is a situation comedy, while Seinfeld simply isn’t – since each week the writers of The Young Ones drew on the formulaic premise of four very different housemates sharing close quarters, a hippy, an “anarchist,” a punk, and a self-interested capitalist. So, while the episodic variations were extremely unusual, everything flows naturally from the initial parameters. (Eg; Vyv accidentally strikes oil in the basement while breaking shit up for the sheer hell of it, Mike claims it as his own and sets himself up as an oil-baron dictator, Rick plans to overthrow the ‘government’ through violent revolution, and Neil frets about the environmental impact and hopes to bring about a desirable resolution through “good vibes.”)
As a side note, in one sense The Young Ones isn’t a situation comedy – it’s a “variety show.” This is really just a ruse, though: The producers wanted to work in stereo, and the BBC had an inflexible policy that situation comedies should be mono, while variety shows should be stereo. They couldn’t get around this, so they decided to include a musical number in every episode and say “Oi! It’s a variety show! Now give us our stereo gear and bugger off!” 
Anyway, what distinguishes Seinfeld is that it never relied on a formula to drive the direction of the plot. This is why it’s a show “about nothing.” Yeah, you’ve got distinctive characters, but there was nothing about their situation that the writers used to hang plot ideas on – unlike a show with an actual premise, like, say, a guy tells an uptight landlord that he’s gay so that he can share an apartment with two girls, wherein the episode writers riff on that situation for laughs. How does he have a personal life without his landlord finding out that he’s straight and evicting him? What about the sexual tensions that arise in this platonic arrangement? You can do paint-by-numbers episodes.
With Seinfield, there simply was no situation like that for the writers to keep rehashing. While, personally, it never really turned my crank, it really was something new as far as American half-hour comedies was concerned. It tossed out a fundamental rule that every single other episodic comedy before that treated as fundamental. It’s the comedic equivalent of atonal music. Seinfeld (and LD), like Schoenberg, surprised people by disregarding an established starting point for composition. “Key? What key?” “Premise? What premise?”