What's the difference between a story idea and a speculation...eek. The problem with this whole discussion -- perhaps the main problem -- is the fuzziness, because it *hasn't been defined yet*. We are now in the OF defining it via this exchange. It's what a person might choose to sue over. And there's no way to quantify or predict that.
I do think there's *some* difference between the two. I think that primary way this discussion can get into trouble is in the level of detail. It's pretty much impossible to sue for a brief *idea*; but the more elaborate a suggestion or speculation gets, the more points of potential comparison emerge, and the greater the liability. "See, your honor? My speculation on what happened contained 57 specific plot points, and what Straczynski later produced had 54 of those points. I submit that he saw what I wrote, decided it was better, stole my idea, and took it for his own!"
But since it has come up...again, I'd say that the main point of differentiation is in the level of complexity. The longer and more detailed the speculation, new histories for characters, new behind the scenes stuff, plot complications, motivations, on and on -- the more something is built that one could sue over.
This is a very difficult area, and there are all kinds of ways that it can bite you. Over on CIS, for instance, Paramount came down on a series of messages with speculative plot info on ST 7 with lawyers and in injunction and major-leage threats against the people involved, CIS itself and anyone else they could think of. We're trying to find a more personable way of handling this situation. And for the most part, I think we're doing fine. But every so often this little issue raises its ugly head, and has to be dealt with.