Well, I heard a lot of this before now. While it could be wrong it was simply abbreiviated into the simple fact that Gaiman and Mcfarlane continue to have slap fights over rights from the 1990s, not getting along at all nowadays despite having both once been partners. Gaiman wants to use one of his more successful hero characters Angela, which happens to be part of Mcfarlane's "Spawn" Universe which he still makes money off of. The central issue being that if this is the same character, meaning not simply one that looks and more or less acts the same, by using it in Marvel an arguement can be made that arguably Mcfarlane's major continiuity becomes yet another one of Marvel's parallel worlds, or at least a version of it does. A point which is paticularly interesting given that Marvel as a whole has never really done much with the concept of angels or heaven, sure we have demons, devils, and all kinds of nasty hellscapes, but other than characters that look angelic and might have claimed to have started the legends early on, I don't think we have ever seen a Marvel character with a directly heavenly origin, at least not one that I can think of.
Bob might be going somewhere else with it, but that is the gist of the situation as I understand it.
As far as the popularity of Angela and so on... well I'll say that a lot of people, Bob in paticular, seem to be getting too heavily into the whole "90s suck" vibe here still. Angela was a pretty popular character in her own right, and the simple fact that she was going to be making some bit apperances in various Spawn cartoons, or that there was a character who was supposed to be her in live action Spawn (never really named) were of course big news among comic geeks. Yes, she's ridiculously hot, and wears some interesting costumes, but really the people that have a problem with that tend to represent the actual minority.
Speaking for myself, I'm all for Angela's appearance in Marvel as it could introduce some concepts that haven't been heavily explored in that universe yet. I also admit that as much as I liked Spawn, I've had a sort of hate on for Mcfarlane since he went after Palladium (RPG company) years ago and forced them to change the name of their "Nightspawn" game to "Nightbane", claiming that the work was too similar to spawn and could cause confusion based on that name. One of the more "WTF" victories I've seen in these kinds of incidents. The way Todd has slapped around the little guy, I'd feel there was a bit of karmic justice if he somehow lost his most famous work/shared universe to a megalithic company like Disney/Marvel much the same way.
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As far as Neil Gaiman's chops as a writer, it mostly comes from "Sandman" which arguably put DC's "Vertigo" on the map. He also did a seres of novels "American Gods", "Anasazi Boys", etc... which seemed to go over pretty well.