For me and for how SON chooses which series, daytime and primetime, to cover it boils down to this...

A soap opera is a serialized drama that tells stories that aren't opened and closed from episode to episode. A plot point that takes place in season one can cause repercussions in season five. CSI isn't a soap opera, but The Good Wife can be. In one aspect it deals with open and closed cases from week to week, but the story of Alicia's marriage and her husbands incarceration and how it effects her personally along with her kids continues on each and every episode. Desperate Housewives is a soap opera as it follows one storyline throughout the course of a season, but the show itself believes its a comedy. Who's right? Grey's Anatomy is a soap opera, but they consider themselves a medical drama series that deals with cases that are open and closed (generally) from episode to episode, but they have characters that deal with romance, ups and downs, deaths, etc over the course of the series' life span. The term soap opera is a gray area as there are numerous programs out there that could be classified as a soap based on what I consider a soap to be, but the show doesn't want to fall into that stigma, because if they do it might turn off viewers who just want to watch an episode today and skip the next week without feeling like they are missing out on a major turning point in a story.

Given the overlapping definitions, I wonder if daytime PTB wouldn't have benefited from insisting on having daytime shows referred to as 'daytime serials' rather than 'soaps' and comparing themselves to 'prime time serials'. I don't think that would have stopped the ratings slide in itself, but I have to wonder if it would have helped daytime writers take the genre seriously. I copied this from the thread on JER and McTavish's 2003 return to daytime:

"No, soaps don’t have to be written on a fourth grade level for the audience to understand them. (In fact, that school of thought combining with the increasing educational background of their audience is causing a major disconnect between viewers of soaps and the folks that produce them..."

At some point, soaps starting parodying themselves and soaps-as-drama seemed to being to mean soaps-as-'melodrama' to daytime writers. Since it's always been my primary form of entertainment, I have to say that I fully believe that the daytime serial has the potential to offer far more to the viewing audience than prime time serial does. The Good Wife is the one prime time serial I've seen that I thought would make a great daytime serial. I want to see more of 'Glee' but I could see Glee as a successful daytime 'soap' as well.

Soaps have to to be domestic dramas with OTT melodrama for the most part. You can have stand alone films be soap operas because of their content (Written On The Wind, Imitation Of Life, Valley Of The Dolls, many others) and you can have serialized movies and yet they still are not soap operas (The LOTR trilogy, the Star Wars trilogy). Darth Vader revealing his big secret to Luke in Empire Strikes Back is the stuff soaps are made of, but the occasional moment does not a soap make.