After watching the John Carter of Mars trailer, I decided to revisit those books of my youth and was happy to discover that they're available in anthology format at Amazon.com for the Kindle for $0.99. Since you can't go far wrong buying any book for $0.99 I went for it and found that I did still enjoy them.
One thing that I noticed was the book length. A Princess of Mars was first published in 1917 and weighs in at about 160 pages. In contrast to this, a more recent novel I've read was A Game of Thrones which is 720. The interesting thing, to me, was that about as much happens, in terms of major plot events, in both novels. I've noticed this trend and many (but not all) of my older books: Witch World is 222 pages, The Hobbit is 320 pages and both have a lot happen over that span. Newer books tend to be much larger; all of the "Song of Ice and Fire" books are over 700 pages. Modern publishers require that manuscripts be a minimum of 100,000 words (about 300 pages) to even consider a submission.
I find this interesting since, in the age of television, video games, and twitter, one would expect readers to be demanding shorter, crisper works. Not that I mind the longer works, I do enjoy the greater depth and detail, and more characters (Princess of Mars has one point of view character, A Game of Thrones has seven, if memory serves). Perhaps modern readers feel that they don't get their money's worth in shorter works. I think it is also true that the craft of writing has advanced somewhat -- in reading The Death of Christian Culture one will be convinced that it has certainly declined in some respects, but I do find that modern works of science fiction and adventure novels tend to be better.