Players that stick to the bitter end; and
Players that do not
This is in regards to the idea of long term campaigns. Those elusive white whales that many players would love to see, but which few actually do: you know, the kind where you start at level 1, and then by means of normal rules progress over 1-2 or more years to level 20 (or in the case of classic D&D go for as many years as you can stomach). Then rinse and repeat!
The players that stick to the bitter end, in my experience, tend to fall into these categories:
Old friends within the group; if you know each other and have known each other for a long time, you are more likely to keep gaming together (so long as you also have a GM who offers a consistent time and space for it);
They can put up with each other; I don't say "tolerate" because when you're friends that terminology doesn't quite fit...tolerate is what you do to people who you must associate with but not necessarily by choice. The friends in the long group may have quirks and eccentricities, but you put up with those because you all actually enjoy meeting and gaming together far more than any annoying traits you may have.
It might be possible to achieve a long term group without these traits, but you likely will run into some problems, including high turnover (a group that runs a long term campaign may have players come and go, but if you reach the end of the campaign and none of the original players are the same you are faced with a Ship of Theseus scenario). You could theoretically have a group go for years without being friends but that's...well....that's pretty sad, actually. If you spend 4-6 hours at least once a week gaming with people, you ought to get to know them as humans and not just players, right?
Players that do not stick with it are both more numerous and easy to quantify. They include the following base groups:
They don't find the group style/personalities a good fit; this is easy, and applies to anyone who for whatever reason decides that spending the next X number of years gaming the same game with the same people is not gonna fit their needs.
The reasons this happens can be varied. It can include these traits:
Short attention span (I call them "buffet samplers"; I admit that as a player I am one of these)
Personality quirks (can't stand --or be stood by-- the rest of the group)
Confused about the idea that gaming is more than just a sport or activity (people who don't realise that making friends with your fellow players is important)
Earnest but time limited (the saddest group, who are restricted from enjoying the hobby due to mitigating life factors but really wish they could be present; a subset of this in today's age is the "I want to log in by my internet is crap" crowd)
Needless to say, to get that campaign you always wanted from level 1 to level 20 with at least enough core players throughout the experience to reach level 20 then you need plenty of the former type of player and fewer of the latter types. You also need a GM who's going to pull it off. GMs have their own range of issues as well, which I tend to quantify as follows:
GMs who can pull off long term campaigns tend to be:
Focused (can stick to the same game and campaign for the long haul; this is harder than it sounds);
Dedicated (realize that they must eschew distractions);
Rewarding (must both feel rewarded for the dedication and recognize that a long term campaign must also reward the players with relevant plot details and interactions that make their characters relevant over time);
Consistent (simply put: you show up at the same time, same place, and run the same game, preferably weekly; just getting this down alone will at least insure you game weekly, even if you can't quite manage long term campaigns).
GMs who can't pull this off tend toward the following:
Unfocused (likes to play the New Game of the Week and tends to understand short-run scenarios and mini campaigns better; or has a vision which is best realized in 10 sessions rather than 60);
Easily Distracted (the GM has too much going on and can easily get distracted by other systems, ideas, or unrelated stuff);
Unrewarding (the GM falls into storygame traps such as railroad style adventuring or enters the campaign with a plot conceptualization that did not leave room for players to enter into and change things by their deeds and action; tricky, because the GM may feel rewarded then wonder why her players abandoned her);
Inconsistent (no regular time and place for a game, no dedication to insuring it happens like clockwork; this of all things is the failing of most GMs; you need the discipline to show up at the same time and place for months or even years).
Without a GM who holds the good traits and none of the bad, a campaign will never last six months let alone six years.