I'd like to go back to the post on family gaming and the follow-up where I contrasted hardcore gamers and casual gamers. The implication in my post is that family gamers are casual gamers. They are, but they are a distinct subset. Two distinct subsets, in a way, since Devin brought up in a comment the distinction between playing with distant family members during a holiday visit vs. playing with close family members on a weekly basis. Playing Life, Payday, Operation, or Monopoly with cousins at Christmas is pretty much identical to other forms of casual gaming, although it does have a little twist.
The goal of casual gaming is, as I suggested previously, entertainment. It's a way to pass the time. A casual gamer may have some favorite games, but is not invested in the game itself. The goal of playing a game with relatives during the holidays is entertainment plus group activity; it's a way of reinforcing your identity as a family member. For family members who have a regular game night, the group activity aspect is the dominant goal; any game designed for family play has to be subservient to that goal of reinforcing the group.
Games aimed at the casual gamer are competing with other games *and* other media, like television; games aimed at the family gamer are competing with any group activity, such as crafts or conversation. Games that create rivalries and risk complaints of unfairness are thus at a disadvantage against more family-oriented games. Consider this: there is a rule in Monopoly that hardly anyone uses (the auction,) abandoned precisely because younger children may feel that it is unfair.
The basics of D&D actually could be a family-friendly game, since famously it is more of a cooperative game than a competitive game. However, there are threatening elements: the Dungeonmaster and the thief. A traditional DM plays the role to the hilt, stocking dungeons with an eye towards tricking players and hitting them with surprises; this could potentially stir up accusations of unfairness. Similarly, if the thief class is included and played the way we sometimes see among the hardcore (stealing from teammates or outright betraying them,) there are going to be a lot of hurt feelings.
Either of these issues can be worked around in a number of ways, but it might be better to admit that the default "family" D&D should be a little bit different from what we consider "real" D&D, with a full DM or interparty rivalry being options for more advanced play.