(or: "Where Did Everybody Go?")
When you look at the idea of the Defenders, it seems great--not just great, but downright natural, an inevitable companion-book to Marvel's long-running Avengers series. If the Avengers is a group of socially-acceptable friendly heroes that fight menaces too big for any one hero, the thinking goes, then the Defenders are a group of socially-unacceptable loners who do the same! On paper, this works well. Doctor Strange's mansion forms a perfect group headquarters, the team can go anywhere through mystical teleportation, and the rest can all be cribbed from the Avengers.
But read between the lines, and you'll find a question that a variety of writers have struggled with for years: Why would a group of socially-unacceptable loners stick together to fight crime?
A team dynamic isn't the same thing as the dynamic of a solo book, particularly when assembling a group of characters who already have their own titles into a team. The readers expect consistent behavior between appearances of a character; if the Hulk is an anti-social misfit who just wants to be left alone, the Silver Surfer thinks of Earth as an insane asylum he's trapped in, Namor hates the human race with a passion, and Doctor Strange is constantly dealing with things that would make your eyes dribble out through your nostrils, well...people are going to expect them to behave like that when they work together, too. Which means, natch, that they won't.
Steve Englehart struggles with this constantly in the first volume of 'Essential Defenders'. Over and over, we hear phrases like, "Just this once," "only for a minute", "one last time," "only becase the menace is so great", all deployed as reasons to get the various members of the classic Defenders line-up to actually team up together to defeat an enemy instead of flipping each other off and walking away. The team dynamic here is working to make it harder for the writer to tell stories, because in addition to coming up with a plot, he has to come up with a reason for the heroes to come together each time. By the end of the first book, Namor and the Silver Surfer have both left the team for good, replaced by less well-known (but more team-oriented) characters like Nighthawk and Valkyrie.
In the second book, without being saddled with a group dynamic that actively works against the writer, new writer Steve Gerber is able to focus in on questions of tone, turning the book into a battle against off-beat menaces that the Avengers wouldn't necessarily be able to cope with. This new dynamic, which turns the Defenders from "misfits and loners" into "counter-culture super-heroes", is much more sustainable (although note that later, after Doctor Strange leaves and the series becomes entirely about B and C-list heroes, it's finally cancelled.) Subsequent attempts to revive the Defenders have all attempted to revive the "classic" line-up of Namor, Hulk, Doctor Strange and the Silver Surfer, and they've all failed because fundamentally, it's a bad team dynamic--anytime the writer is struggling to keep all the characters in the same room, it's going to be hard to turn them into an open-ended, long-term series.