The last few D&D games I have been part of have all been "open" games. By open I mean that they are played with varying participants. The Northern Marches campaign was specifically designed to be open. It was created to accommodate busy adult lives and was designed so that it did not matter who showed up to play from week to week. The 4E games I am running at the local meetup are by necessity open and the skype B/X game is open in that I just made an open invitation for people and accepted a couple more than I expect to be able to make each scheduled session.
An open game contrasts with a closed game which I think of as how I have previously mostly played D&D in that I was part of a set group. Closed groups may or may not play the same game from week to week but they a fairly consistent in who shows up to play.
Whether a game is open or closed has an impact on social aspects of the game, the campaign design and the game mechanics themselves.
1. Social - a closed game has a slightly different social element than an open game. This is pretty evident since in an open game you never really know who might wind up sitting down at the table. I have found two interesting social dynamics in the open games I have run. The first is that I had the first fight between players during a Northern Marches game. The players did not know each other and it stemmed from a roleplaying incident where a character reacted to having another character accidently gunned down by friendly fire. I don't think that the reaction would have been the same if the players knew each other and had been party of a closed game. The second, an honestly one I tried to cultivate for the Northern Marches, is a bit of competitiveness between players. I wanted to try to get people to want to play frequently by giving them FOMO (Fear of Missing Out). If they new that another player's character found a magic sword during a session they were not able to atend maybe they would be more inclined to show up the next session.
2. Campaign Design - If a DM doesn't know who is going to be at his game, it makes it tough to carry on a plot. I think this leads to one of two reactions from the DM. Either they railroad the adventure to make sure that their "story" gets told or they adopt a sandbox. A sandbox doesn't have an overall plot so continuous attendance isn't required. An open game also encourages more episodic play with each session being more self contained.
3. Game mechanics - I find that closed games lead to more house rules. If you have a long standing group, everyone gets to know preferences and can take the time to learn house rules. Open games make communicating a long list of house rules more difficult.
What do you think? Are there other aspects that are impacted by open or closed games? Are there advantaged to open or closed games? Which do you prefer?