So, what does everyone think is necessary for a good harem story. For the purposes of this discussion, the harem needs to be a central plot point if not the focus of the story. Corrolary to that, the females should be developed unique
-A reason for the harem. If there isn't a reason for a relationship like this, the story will probably fall apart. The reason can be pretty much anything, from political marriage to keep the kingdom from falling apart, to a mystical bond that ties them together, or even just being stuck on a desert island with only one available bachelor, or just a major imbalance between the sexes.
-Size limitations: The bigger the harem, the more difficult it is to properly develop all of the female characters, and the more it stretches credulity. This can be alleviated somewhat by dividing them into tiers, (primary, secondary, and tertiary characters,) but the further away the girls are from the spotlight, the less impact they have on this discussion.
I would think that for the most part, you don't want a harem bigger than five girls. That gives you enough variety without risking repetition, and keeps the pace of the story from dragging too much. It also gives you a huge number of secondary relationships to explore between the girls, without making it overwhelming. Five girls and a guy give you 30 individual story lines between two characters, which should give you plenty of aspects to explore
-A reason why it's accepted. Most people want to be the one and only in their partner's life, so we need a reason for them to accept something different. One of them is because the protagonist is too wishy washy/dense to choose a girl, so the harem exists in a weird quasi relationship state. This is the form seen most commonly in anime harems, and while it isn't a proper harem, generally ending when the hero finally chooses someone, it still has many of the dynamics. Other options include political necessity, cultural differences, or even just keeping it secret (though if it's ever found out, there should be some fallout.) Whatever the reason, there has to be some justification within the story for why people go along with the harem existing.
-A decent pace of development. You can't just go straight into them being in love. There is a degree of adjusting to the unusual situation, getting used to your new family, and the like. There probably are and should be some dissent and growing pains, since that is what makes a story interesting. Love at first sight can happen, but there should be some realism in that just loving them doesn't make everything perfect, and your certainly don't know everything about them.
Probably missing a few point I wanted to make but I'll come back to this later. For now, this should do.