Vol 6 is Essentially (if you pardon the pun) the last volume of the Defenders series. It's true that the entire book ran for an impressive #152 issues considering that it wasn't one of Marvel's heavy hitters but it's easy to consider this the final volume. One way the book gives the feeling of a final volume is from wrapping up several ongoing plot threads. The Elf With The Gun (originally just supposed to be a way of showing the acausal nature of life) is addressed, the fate of Nebulon (who menaced the Defenders back in vol 1, 3, and 5.) Hellcat finding out more about her past (from vol 5) Finally having the Defenders fight the Enchantress again (a plot thread all the way back from the vol 1) All these plot points and more were addressed and longtime readers were richly rewarded.
The Defenders did something a lot of successful team stories do. They focused and developed on lesser known characters that weren't able to support their own books. Nighthawk was originally some poor Batman parody back when he appeared in the Avengers but over the course of the Defenders he got his own interesting back-story and became the heart and soul of the team. Sometimes acting as the straight man in absurd situations the group got into. Dr. Strange was always the wise team leader. Valkyrie learned about being human, Damien Hellstrom struggled between good and evil etc.