Actually I think is a pretty interesting idea for a thread, but maybe that's because I tend to collect those heroes of yesteryear that no one else seems interested in any more.
I'll try and get things started with Tarzan. ERB's Jungle Lord was probably one of the most recognizable fictional characters of the previous century. He was the subject of numerous books, movies, TV shows, comics strips and comic books, promotional items, you name it. But today Tarzan is nearly forgotten, the last major appearance of the character in popular media being the somewhat mediocre Disney animated movie from a few years back.
As for myself, I remember a much better animated version of the character - Filmation's Tarzan: Lord of the Jungle which aired on Saturday mornings from 1976-79. This was my first introduction to the character as a little kid and I thought it was the coolest thing in the world. Next I discovered the Marvel series and then the original books. As a child I had no awareness of the inherent racism and social Darwinist ideology that the character represented. To me Tarzan was the ultimate hero - always brave, always noble, always the good guy, without the slightest hint of moral ambiguity in his actions.
When I got into collecting a few years it was probably inevitible that I would start picking up some early Tarzan comics. The first Tarzan comic strip appeared in January 1929 and is tied with Buck Rogers as the first real "non-funny" adventure strip. The B&W strip, which adapted the first Burroughs novel, was drawn of course by the legendary Hal Foster. Foster would return a couple of years later as the writer and artist of the Sunday color Tarzan strip, which even today has got to be considered one the greatest comic strip runs ever.
Tarzan's first appearance in the comic book format was in the first issue of Tip Top Comics in 1936 which reprinted the Sunday Foster strip. This feature ran in Tip Top for a number of years.
The first all-Tarzan comic book was the oversized Large Feature 5 (1939), which reprinted the complete Foster B&W 1929 strip:
The following year saw the release of Single Series 20, which was 64 pages of all-color, all-Foster Tarzan strips:
These last two books were once major key grails, but have not kept pace with the big super hero keys over the last few decades.
I would say that there are several reasons for the character's decline. When Tarzan of the Apes was first published in 1912, the jungles of the Congo were largely still unexplored by Westerners and still represented a mysterious frontier. The existence of the mountain gorilla had only just been confirmed a few years before. But today the Dark Continent just doesn't have the same mystique. Rather than a symbol of mystery and adventure, Africa has become a symbol of tragedy and exploitation. Tarzan is a fictional product of that Western exploitation - a physical embodiment of the perceived manifest destiny of European colonization. Because of this, the character has become an embarassing reminder of the sins of the so-called "civilized" world. Perhaps there's no more place for such a character.