Karen: Not too long ago, in our discussion of the Silver Surfer, some mention was made in the comments about whether or not the Surfer really worked all that well as a lead character as opposed to being a team member or guest star. There are a number of characters that have had their own titles that we could also throw into this category and discuss the merits of whether they truly deserved or could carry their own book. I would actually say that two of the Surfer's fellow early Defenders -- Sub-Mariner and Dr. Strange -- also may work better as supporting characters. (Regular poster William also had the same idea in our comments over at the Surfer post!). Neither have been able to maintain their own titles for long periods of time. With Sub-Mariner, I think there are certain factors that go against him: his anti-hero, mercurial nature, and the whole 'ocean kingdom' motif, which seems to be a hard sell. Dr. Strange is a bit harder to explain; in the hands of certain writers and artists, he's been an interesting character, but all too often he's been in less than inspiring stories. Being a magical character, he also has the problem of poorly defined powers, which enables him to overcome situations a little too easily at times. However this same attribute often works well for pulling other characters' fat out of the fire.
Doug: I'll confess to never having read more than a handful of Silver- or Bronze Age books with Namor or Dr. Strange as the headliner. I mentioned a couple of weeks ago that I have read the Surfer's 18-issue run, and enjoyed his revival as a solo star under Steve Englehart and Marshall Rogers (later Ron Lim). It benefited, though, from numerous cosmic guest stars and the breaking of the Galactus-installed barrier. But as you allude to above, I enjoyed all three of them in those early issues of the Defenders.
Karen: In fact, at times I have even questioned whether or not the Hulk is a good lead character! I think that title has often depended on the quality of the supporting characters in it to carry it. The Hulk himself, particularly in his childlike version, has just not had enough depth to carry a title by himself most of the time. You have to have an interesting group of characters around him, or guest stars at least, to keep things going in that case.
Doug: Well, I'll also confess to only having read a handful of Hulk comics! You hit the nail on the head as far as I was concerned -- I just didn't find the character interesting solo, but again was wonderful with the Defenders. Naturally, it was a gas whenever he showed up in an Fantastic Four story, too. Your comment about solo books needing significant supporting cast members is spot on -- Spider-Man, anyone? I'll even go so far as to suggest that my interest in Captain America waxed and waned over time.
Karen: I'm not as familiar with DC but I would wager that Aquaman, in his many incarnations, might fall into this category. What about Wonder Woman? She's had trouble supporting her own title. I thought she was interesting when George Perez came in and built up the mythological angle but it was still the infrastructure around her that became interesting, not the character herself so much.
Doug: I can't comment on Aquaman, other than to wonder why he'd fare any differently than Namor (who I find to be a lot more interesting). What about J'onn J'onzz? I think he's a great JLAer -- very cool (and cool-looking) character. But I don't know that he could really swing a solo book. Perhaps in that regard he's like the Vision. As you said, I did like Wonder Woman under Perez's handling, but couldn't get into any stories written prior to the Crisis. Is that just a WW thing, or is that my greater prejudice (with some exceptions, certainly) to Silver-and Bronze Age DCs?
Karen: Time to add your candidates to the list: who are the heroes who really should be staying off center stage?