The IT Crowd is a show about a group of fools getting into ridiculous situations at work. It’s a good laugh and has some classic moments. My partner doesn’t make or get tv/movie references, but even she can drop references from it into casual conversation.

Big Bang Theory is a bland comedy that wants to be much more than it is - relationship drama, social exploratory, situational comedy. But it tends to fail 90% of the time, and the closer it gets to ‘relationship drama’ the more grating it gets.

I haven't watched much BBT, so grain of salt, open to being called out for mistakes.

I'd say The IT Crowd was more centered about a workplace, even if many of the jokes are about their romances, hobbies and personal life, and even if BBT had workplace episodes. The central set of IT Crowd is their basement office, the central set of BBT is an apartment. And I don't think that's a trivial difference, it changes the kind of humor and how relatable much of it is.

Also, a big gripe about BBT was the laugh track. It's normal for sitcoms to have canned laughter, IT Crowd had it too, but BBTs is uncomfortable. Like trk said in another reply, "If there was no laugh track you wouldn’t even know jokes are happening." There are plenty of BBT No Laugh Track edits on YouTube and it highlights how much better it could be without the forced laughter.

Finally, BBT is a lot more US humor than British humor, not to say that as point in itself, but I feel like it makes the BBT feel to me a lot more like "laughing at nerds" despite that being a major factor in BBT. I also feel that Moss is played more wacky and exaggerated than (say) Sheldon, who certainly has strange peculiarities but is played more subtly or realistically, which I feel makes their character more mocking than absurd parody.

(also I just like the theme music much more)

Comparing The IT Crowd to The Big Bang Theory is like comparing Pavarotti to a braying lamb.

BBT is the braying lamb, for the avoidance of doubt.