One of the greatest barriers to the growth of online gaming is the inability to pause.
For all but the most fanatical of gamers, our non-virtual lives take precedence. If the phone rings, we answer it. If a family member needs help with something, we get up and help. There are a thousand circumstances which might beg a gamer to stop playing, for a moment or for hours, and those things are more important than entertainment. Sure, we try to be considerate to the people we're sharing that entertainment with, but entertainment is almost always a low priority for responsible people.
Is it possible to break past this barrier? Is it possible to design online multiplayer games to be more forgiving of surprise breaks without making gameplay turn-based and avoiding real-time engagement?
Unfortunately, it seems there's no strategy that works for all games. Different styles of gameplay allow for different methods.
Time and penalties
Two methods are being adopted by an increasing number of MMO developers.
The first is to shorten play-experiences into smaller segments. Even the longest adventures can be broken into more accessible chunks. The problem with this solution is that it usually breaks dramatic tension. There's less time for build-up.
The other method is to reduce penalties for failure. To use an extreme example: if a player's character could simply be resurrected on the spot, after the character died during the player's absence, then the only penalty for "pausing" gameplay was the group being short a combatant unexpectedly. For games that primarily appeal to a sense of accomplishment (most MMOs), this undermines the strength of that accomplishment.
Focus on the moment
Another method is to make action its own reward. Consider replayable FPS games, like Halo 3 or Star Wars: Battlefront. There's certainly some appeal to accomplishment, but the heart of gameplay is the thrill of combat. There are enough dynamics that one battle doesn't feel exactly like another. The player's focus is usually on the moment, rather than the goal.
If the goal is secondary to the experience, then failing to meet the goal because you had to leave the game abruptly isn't such a big deal. If I die in Halo, oh well; I'll just try again. Obviously, the previous method of penalty reduction plays into this; but penalty reduction doesn't undermine gameplay as much when the conclusion of an encounter isn't as important as the encounter experience itself. Thus, I think it's ultimately a different method.
The main casualty of abrupt leave-taking in MMOs is grouping. If you leave suddenly, your group is left with the consequences. A focus on the moment reduces those consequences significantly, since failing doesn't severely affect the group any more than it affects you. But this is where the style of a particular game becomes important. In a level based game, you can't simply rejoin the group at any later time, as one could in Star Wars: Galaxies (not counting shuttle time).
As I said recently on Craig's site, I think the best games combine experiential gameplay with achievement-focused gameplay. More emphasis on experiential play is advantageous for a number of reasons, but one reason for the emphasis in multiplayer games is the leeway for pausing. Even if a game is based on levels and progressive goals, enabling the player to focus more on momentary experiences will helps for brief player-leaves by making the replay of an encounter less annoying.