Matt Neuberg over at Tidbits has noticed an interesting behavior in Lion: Occasionally, apps running without an active window on will disappear from the Command-Tab application switcher, and sometimes disappear from Activity Monitor completely. The culprit is a new "feature" in Lion called Automatic Termination, which will shut down and quit any (compatible) apps that aren't active and don't have any visible windows. Of course, most official Apple apps support Automatic Termination.
As Neuberg notes, this is obviously a hat tip to iOS and its background processes, in which you "close" an app simply by navigating away from it, leaving the system to shut it down when additional memory is needed. Neuberg suggests, however, that this implementation isn't so much a feature as a problem. Obviously, a desktop computer has many more resources to run processes than a mobile device, and there several scenarios in which a user might need to access an app that's been inactive for a while.
Granted, with Lion's Auto Save and Resume features, you could argue that there's no real difference to an app or a user between "inactive" and "closed," but Neuberg says that there is, and he doesn't like it. The feature is designed to keep the decision of which apps are open and closed away from the user, and while that might be a help on iOS, it's a hindrance on the Mac. I agree -- I'm perfectly capable of dealing with app management on my desktop, and it's lame of Apple to take that away from me.
Of course you can't opt out of Automatic Termination, short of simply avoiding compatible apps. There are certainly good reasons on both sides for having or not having this feature running, but in this case, it seems like bringing over this certain aspect of iOS app management might not be ideal for all Lion users.