Novels and irony go together, in the sense that many of the literary techniques that work best with novels involve some kind of irony, namely, laying down a limit while also transcending it. This always creates some difficulty in adapting a novel to cinema, because cinema does not do irony very well. It's not that you can't have ironic cinema, but there are fewer ways to express it, because the irony of novels arises from suggesting; but in film, suggesting is easily lost in showing.
This problem is intensified when it comes to adapting an epistolary novel, because epistolary novels are even more suggestive in character than ordinary novels. In addition, an epistolary novel by its very structure is related to a dialogue, and dialogues do not easily translate into the spectacle required by cinema.
Thus an adaptation of Austen's epistolary Lady Susan, as Whit Stillman's Love and Friendship is, faces a very large number of problems at the outset. All in all, they are masterfully handled, though. One of the things Stillman certainly did right was to keep the pace fairly swift; the movie is only 91 minutes long. Stillman also makes excellent use of the snippet, that is, the brief scene letting us know something is happening without spending a lot of time dwelling on it, just allowing the snippets to interact by juxtaposition and contrast. Since much of the action of Lady Susan is either suggested or briefly described, this is almost certainly the best way to handle the problem.
Humor is more difficult. Austen's humor is very ironic and sometimes very dry and subtle, as well; there's no way to represent it adequately on the screen, which is why Austen adaptations always run the risk of being too serious. Stillman mostly cuts the Gordian knot with this problem; despite the fact that Stillman manages to be relatively faithful, a lot of the humor in the movie is Stillman's rather than Austen's, and it is inevitably less subtle, since we at times are simply told the punchline, which is then flagged by all the arts of cinema. (Cinema inevitably has more ways of directing our attention to something than any book could possibly have.) This is most obvious at the end of the movie. Austen can simply leave open a spectrum of possibilities; the movie has to show us something definite.
But the humor largely works as well. The Fourth/Fifth Commandment joke running throughout works surprisingly well, and does double duty as an example both of Lady Susan's manipulativeness and as a proof of her power to affect people. Sir James Martin's simpleness is almost absurdly exaggerated, but since everyone else mostly plays the humor straight, it's nice to have a contrast. A goof gets the laugh going, and that gives people a running start on chuckling at the witticisms. In essence, Austen's humor is being blended with British sketch comedy. And the result is, I think, the funniest Austen adaptation ever made.
The chief difficulty with adapting Lady Susan is that Lady Susan is actually quite malicious, and we can see it because we are getting her candid comments along with the reactions of other people. But this is easily lost, and inevitably much of it is lost here, because film, again, cannot so easily build a wall to hide something and then just give us clues about what's on the other side. We need Lady Susan's perspective here -- but we only get it on screen through pretty Kate Beckinsale sweetly saying things that we know would be shocking but which mostly come across as charming and funny -- as indeed they would have to, if Lady Susan actually said them.
But perhaps this works well in its own way; Lady Susan is a liar and manipulator, a villainess to the core, and yet in some fashion she shows up here as better -- certainly more competent and charming -- than many of the heroes and heroines Hollywood throws at us and somehow expects us to admire.