There's this widespread belief that "classics" are altogether awesome books, untouched by time, that can be appreciated by anyone in any situation, or by people with all sort of tastes.

People who've read a lot of older literature know that's not quite true of most of the classics. Not all of them are masterpieces certainly, and even those often need context and some work (occasionally, much work) to be fun or even productive to read. To some, this "work" is recreational, a pleasure, and to others it's just work, or something they can't be bothered with. That's fine, to each his own, but to those for whom it's of no interest at all, many older books are simply a loss of time that would be better spent on other literature more to their taste, or even on renting the movie version...

Most of these "classic" books don't fit well the modern expectations of narrative fiction, but that doesn't make them "horrible". Some, a few, transcend their dated form, or have a particular appeal to specific people for all sort of reasons, that they don't need much more than themselves to be appreciated today, but a lot of books considered "classics" don't, because they've not become classics because of their timeless and "universal quality", but for other more specific reasons, such as their influence on the evolution of fiction, their importance on the society or the ideas of their time or on a specific culture, or by being the pinnacle of a genre in time/space, in quality or popularity and so on, or even because their prose is exceptional (or very representative) - a shining example of the mastery (or just the state) of a language in space/time.

A great deal of "classics" lose much of their interest if the modern reader doesn't make the effort to get a bit of background why this work is considered a classic, and to understand the context surrounding the book. Most of the time, "it's a classic and it's a great book" just doesn't cut it. People with no interest in history (especially social history), or the evolution of literature, often don't care much for classics, even when they're great readers, who enjoy a lot books "culturally tailored" to the contemporary sensibilities. There's no shame in that, really. If you put Erikson's GotM in a time machine, 99.9% percent of Dumas' or Dickens's readers would have concluded rapidly they've missed a few previous volumes!

And for some concrete examples: Sartre, a great philosopher and someone inescapable to understand post-war Paris - but an average playright and novelist. There's not much in there today for those who don't make the effort of learning a bit about existentialism, and the ideas and society in France in Sartre's time.

Nadja by Breton. A classic, a must for those who study 20th century literature, but without prior exposition to surrealism and its place and influence, Nadja is a most boring book. The same is true for Alfred Jarry (who's far best read after the later writers he influenced)

Try to get a 10 year old to read a Jules Verne novel. Most won't get past 10 pages before telling you it's boring and nothing happens. By today's tastes, a novelist ought not to spend much time setting up the story and characters before a lot of things start happening. You used to situate your story, and get the reader to develop some attachment for the characters before the story really began. Nowadays (as a famous French publisher/editor of youth literature was explaining in an interview last week) a YA writer is advised that he has 3 pages max to grab a kid's interest, and he can't slow down with the "page turning hooks" before the reader has developped attachment to the protagonist (in her opinion, Harry Potter's success owes a lot to the way Rowling began the story, intruiguing or amusing the reader one each page until her young readers were well attached to her characters). In the times of Verne, or Dumas or Hugo or Dickens, readers wanted context, a lot of context. People who read Dickens didn't go to the slums. Dickens went for them. They wanted and needed elaborate descriptions of everything, in the days when being described something by someone who's seen it was for most people the primary way to learn what everything outside their town or region looked like. The description of a costume would contribute to make a character feel closer to the reader, or on the opposite make them dream of things they would never otherwise see.

Dumas's novels can't really be appreciated if know nothing of the sensibilities of the readers of his time or the context in which those novels were written. Today people moan that his descriptions are useless "filler" because he was paid by the line, but many of his readers read him for those descriptions of unknown locales or past times! They also got a chapter a week at most, so they wanted to be reminded of past stuff a lot. Without a good understanding of the feuilleton genre, Dumas appears to be a terrible writer. Dumas' better known novels are classics because of they are the best and most popular of the French feuilleton genre, and had a massive influence (direct, on novelists in France and abroad, and on the popular culture even late in the 20th century).

Saint-Simon is another "great writer" (one little known by the non-native speakers) who is very demanding for modern readers. His language is beautiful and interesting, and somewhat unique. His court memoirs are considered the pinnacle of the genre, and he remains a must-read for educated Frenchman. But without an interest in and a good knowledge of 18th century France, these memoirs are thoroughly boring (and best read in "best of" editions then... just to get a taste of his extremely efficient vitriolic prose). Saint-Simon is a classic of literature, for no one has quite wielded French with the precision and sharpness of a knife as well as he did. His work is mostly of historic interest otherwise.

Rabelais is yet another. He's a shining example of the French renaissance man, fuelled on classical culture. If that isn't what you're interested in, or in 16th century French (before it got a major face-lift under the Bourbons), Rabelais's books are of fairly little interest today, for all they are classics.

My examples are mostly French as this is my cultural background, but I think the same is true for most of the books labelled "classics". Their "timeless appeal" is usually vastly exagerrated, IMO. It exists certainly, but mostly in the sense that these books have for various reasons an importance in the history of literature or culture, and most of them will appeal to people interested in that, or other book-specific aspects. War and Peace is of immense interest to those who like historical sagas, or in the 19th century etc. but Tolstoy will try the patience of many modern readers. The same could be said of Dickens, who like Dumas, Doyle, Swift, Austen, Shelley, Stoker most people know from TV/movie adaptations (and the same for lesser known classics, for e.g. few outside France and not tons even there still read de Laclos today, but everyone's familiar with the movie Dangerous Liaisons). These are still great stories, but enjoying them as novels today demands certain interests or sensibilities. To a great deal of people, many "classics" are better enjoyed in a modern reformatting (among other things, the "moderns" have the same interest in lavish visuals and "eye candy" that the original readers used to have in elaborate, detailed descriptions)

Of course, there's also the aspect that most of these "classics" have in some way, some time in a very large way, some importance in understanding cultures, where we come from and how - socially, culturally, politically, linguistically and so on. Not every classic book will be enjoyable to everyone, but that doesn't make their reading any less recommended, especially with the modern educational systems that leave so much to individuals when it comes to gain a general culture. The established classics remain a good "map" to general culture, even if the Renaissance ideal of having read "all the important books" is nowadays way beyond anyone's reach.