Truthfully, I think the game industry is getting too greedy and this is a sign to be honest. See one of the things that makes gaming such a big deal is the perception of value compared to what is being spent, despite what has always been massive overpricing. Things like the coordinated $10 accross the board price hike a few years ago, and the desire to get people to pay for simple online multiplayer which has always been free. Subscription fees like those charged for MMORPGs being justified for things like around the clock staffing and support, which in general your typical fighting game, FPS, or racing game does not apply. Nor does that subscription include free content upgrades like for a WoW (holidays, new dungeons, etc...) especially given the mania over DLC which is already being exploited.
Simply put I generally do not maintain a "gold" subscription for XBL except in very short bursts when I have a new game with multiplayer (typically a fighting game). Honestly though I am gradually leaning towards PSN for when I want to do fighting games, because despite early problems with slowdown I think the last couple of updates have increased the quality substantially.
As far as what could get me to pay for an online subscription to PSN, or more regularly pay for gold?
Well, right now I have my internet provided as part of a package deal that includes TV, Internet, and a lot of free "on demand" content as far as TV, movies, etc...
To get me to pay in addition to that, and feel the service was worthwhile, it would ultimatly have to provide services I do not currently receive. Such as free movies and shows that are not currently availible on free "on demand" servies, or simply taking a lot of these DLC games that you pay for and providing them free to subscribers the first month they are out or whatever.
Let's say that PSN offered a free game, and a free first run movie every month along with access to the same services provided by an "on demand" service. So basically instead of charging me a buck or two to watch an episode of a TV show from last season, it would be free. This being more useful to those who don't receive a "package deal" as it is right now. I'd consider that service worth say $10 a month as a recurring subscription. It would be fairly competitive with other services.
As it is right now I find it vaguely ironic that both PSN and XBL try and charge people for content that is free even without such a package deal. For example with just an internet fee (which you paying to access PSN or XBL) you can hop on something like Fear.net and see a lot of the same horror movies at various time for nothing but enduring an advertisement or two, yet your console company wants anywhere from like $2-5 a pop. Sometimes it makes me wonder what these guys are smoking and if the guys evaluating the usefulness of these services and such even consider alternative (legal) ways of getting the same content. I mean anyone on say XBL already has an internet connection (duh).