Dependable RPG Players
I think being a dependable player basically means that the GM and the other players are able to depend on you to show up and move the game forward. Both elements are truly necessary for a player to be deemed "Dependable" In my mind. A player can show up but if they do nothing to move the game forward, or worse yet actively seek to derail the game, they are not dependable. Getting together to play an Role-playing Game , whether in person, via a virtual gaming table, or even a play-by-post forum, the players and the GM essentially enter into a "Social contract" Of sorts. There are certain agreements made, whether formal or not, and when one of those involved fails to live up to those agreements/expectations, they are not dependable and I would not want them in my game.
It is true that real life happens, and there are times where it is perfectly understandable when a player is unable to make it, but when this becomes the rule rather than the exception I don't think you can consider the player dependable, at least in terms of gaming. Even if their reasons were justifiable and they are dependable in other areas of their lives, I wouldn't consider them dependable for gaming purposes (Even if it were through no fault of their own). It is an unfortunate reality that I have been faced with on occassion.
Dungeons & Dragons is four decades old now and many of the players have aged right along with it; where once in the 80's the game seemed to be dominated by teens and young adults in college who may have had a lot of time to devote to this hobby, these players are older now. They have lives, jobs, families, etc...real life happens and takes priority to gaming; and that is how it needs to be.
I hold no grudge against the player who starts a game and then the circumstances in their lives change and they are forced to bow out of the game, assuming they give me a heads up. When I get irritated is when a player starts a game with no real intent of seeing it through, and often times they quit in the middle of things without informing anyone that they are unable or unwilling to continue; they just sort of disappear. It is inconsiderate to the GM as well as the other players at the table (Virtual or otherwise), who have put in effort and time to develop a game/story that involves the player's character only to have the character suddenly without direction. Now the group has to adjust and come up with a way to either explain the characters absence, kill the character off, or the GM has to take the character over as an NPC.
As a GM I know that a lot of time and energy can go into developing a game, creating obstacles and encounters, and developing something of a story based around the characters. A character dropping out can really upset the game, if nothing the balance of the game is off because encounters may have been designed around the party composition and size, which has now changed; or the GM has to take over the character as an NPC and it becomes more work for the GM who has already put in a great deal of time and effort. It's less of an issue in systems/games that are less focused on combat, but I still find it very frustrating, as I'm sure other's do as well.
Play-by-Posts have their own unique challenges when it comes to players that are not dependable. Whereas at a gaming table, if someone doesn't show up you can wait fifteen minutes or so and then move on. In the forums you may go days if not weeks waiting for someone to post and in the meantime the whole game either crashes to a halt or slows to a crawl. It's even worse when the GM bails without telling anyone (As I recently had happen in a Play By Post game on different site).
Having four players with three that post regularly and one that posts something maybe once per week also slows things down significantly and may leave the other three frustrated. This is another area where dependability is a factor. That is why I think it is important that there is an understanding up front about posting requirements and quality of posts; if you are that fourth player that knows you can only post maybe once every other week, then it is not fair to become involved in a game in which the other players are posting daily. Find a game that the players only expect the occasional post lest you be the one that the others see as less dependable.