I enjoy the Big Lebowski very much, and I also see a lot of people who don't like it. Typically the people who don't like it view it the same way you did, just at the surface. If you take no enjoyment from the quirkiness of the characters and just see the movie for the jokes expecting a knee slapper then I can see why people would be unimpressed.
To me it's almost impossible to understand how people can not love this movie. It is virtually perfect in everything it does. People will of course point out the acting which is absolutely fantastic, the rich cast of oddball characters or the deadpan and super dry dark humor. To me though, where the movie really excels is in how the structure of the movie perfectly mimics the story of the dude. Stuff like the constant use of dead ends and lack of payoff makes the story itself seem as bewildered and haphazard as the Dude himself. Some people might see this as 'problems with pacing' or whatever someone said above, but the fact is that the Cohen brothers have put that 'structureless structure' in place on purpose, and it's amazing to be able to keep the movie together while it's basically free flowing. It's a perfect fit to the stumbling of the waaaaaay over his head hero of the movie.
My problems don't have anything to do with acting, pacing, etc. My problem is much more simple: there simply weren't many funny moments. And by "many", I mean more than average. These seems like a reasonable request...I mean, in a comedy, you expect funny stuff to happen.
A lot of people have been giving answers like the bolded statement which are extremely vague and difficult to understand. This isn't directed to you personally as you give a more defined example in the next sentence (though I don't agree that that's funny). I don't really want to talk about "direction" or "charm" any over-arching concepts like that as that's far too abstract and complex. Many of the terms like "charm" are just unsubstantiated buzz words anyway. I'm interested in specific, tangible scenes that people find funny just to get a better understanding of people's rationale.