In past entries about connected games (which I'm going to define as primarily single player games that incorporate online or multiplayer functionality to enhance the single player experience), I've talked about some of the possibilities that connectivity provides:
(I called them "connective" games, since they didn't really have a name then, but as they've become more common, the term "connected" has stuck. So I'll switch to that.)
Now I'm going to talk about some problems that are going to diminish the appeal of these types of games.
Problem 1: Meaningless features
What we call leveraging virality, most gamers call annoying spam. Bribing players with in-game rewards to get them to market your game is not what connected features should be about. While they might bring in additional players through brute force, these anti-social social interactions turn many players off of connected games for good. Who wants to play another Facebook game when you already know that you get more rewards from exchanging spam than you do from actually playing, and that you're going to be required to harass your friends to make progress after a certain point? Even a Facebook game that doesn't do things this way suffers from the (very justified) preconceptions that these other games have created.
Solution: Bribing your players doesn't count as a worthwhile gameplay feature. Make connected features that enhance the game for them instead.
Problem 2: Server shutdowns
This is a problem shared with MMOs and other online games: when the servers are shut down, the game loses part or all of its appeal. Even successful companies often close down a game's servers within a couple years of its launch. These closures damage the appeal of all future games with online functionality, as players now have to consider how long the servers are likely to stay on when making a purchase.
Solutions: Keep servers up even when the population alone doesn't justify it. Promise players up front a minimum length of time that the servers will be available, and make contingency plans (i.e., contracting a 3rd party in advance to maintain the servers) in case of company collapse or other unforeseeable events that would end service.
Problem 3: Screwing over non-connected gamers
It's great to have these experience-enhancing features in the game. It's no so great when you require a connection to play the game at all. I'm looking at you, SimCity (for the first time in months, actually). Requiring a constant online connection to play a single-player game is not going to go over well, whether you're doing it for DRM or for extra features. Especially since you should know that the launch period - the most important period in determining a game's long-term success - is going to stress your ability to deliver a quality experience to players. You need that true offline experience not only for that small portion of your audience who can't or won't connect, but also as a relief valve when your connected service is down or overwhelmed.
Solution: if you have a connected single-player game, have an offline mode.