I just read this entire thread, and it surprises me how many people don't get the fact that Withnail privately told Uncle Monty that Marwood was gay in order to get the keys to the cottage. That fact informs the entire rest of the plot.
The parts of this movie that I couldn't get at first were the words "saveloy" (I'm a New Yorker, so I had to go digging to figure out what that was - it looked just like a hot dog to me), and some of the stuff Danny said during his story about the dude who went to jail because he told the judge "You think you look normal?". The Danny scenes were hard for me because I was watching it on an old regular dvd that was awful - I could crank the volume to the top and still barely be able to hear it. Solved that problem when it came out on blu-ray, because they've revamped the audio... now I can blast the roof off with the soundtrack and hear every beautiful syllable.
I was also confused for a while about Withnail's reply to Danny that "A gang of cheroot vendors considered a haircut beyond the limit of my capabilities." - but finally got that he was referring to the cigar commercial people refusing to hire him, giving him the sorry excuse that his hair was too long for the part.
It's amazing how many things will come to you if you watch a movie a couple hundred times...
Also, if anyone reading this thread has parts of the film they don't get, a fantastic help (and a great read as well) is the screenplay. Bruce Robinson's writing is fabulous and funny, and you get a lot more insight into every scene (plus some that aren't included in the film) from his intros and notes. I carry it with me everywhere.