Foursquare just upped the ante  in its battle for control of the check-in location-based business by adding in the ability to upload photos and comment on friends' check-ins. It's a subtle change, but could have big impact on the battle with peers like Facebook.
Noting that its users have been "requesting these features for months" in its blog post  announcing the new features, Foursquare also remarks that it's been promising to add them "all year" and "really hustled to get it out for the holidays," presumably to maximize all the shopping and social checking-in that'll go on over the next week or so. Foursquare "just became more useful and social" it goes on, adding that some partners like Instagram and Foodspotting have already worked to integrate the new powers into their Foursquare-friendly activities.
With the new powers you can comment on your friend's check-ins as well as your own, which is actually a powerful trick that could turn Foursquare into even more of a chatty network than it is already: Cue lots of comments like "I'm in a different coffee shop around the corner," humorous comments, or direct recommendations of alternative places to shop/eat and so on. Foursquare sees the photos option as even more revolutionary than comments, however. Now you can upload photos you'll be able to "see dishes before ordering them, figure out if a venue looks fun," or even easily "identify a hard-to-find spot."
Of course there are opportunities for abuse of the system--imagine uploading a disgusting picture of a dish which isn't actually an example of a particular venue's cooking--but the benefits are significant, and bad behavior like this is partly mitigated against by the fact that tagged photos will carry your username with them. Photos on check-ins are only viewable by your Foursquare friends, presumably as a form of privacy so that only people you know and have befriended get to see exactly what you're interested in snapping a pic of. But "tip" and "venue" photos can be seen by anyone on Foursquare--as they need to be if they're to act as hints to other Foursquare users when they visit the same location.
The mention of Instagram , another hot-topic web phenomenon at the mo, is interesting: "Photos from Instagram pushed to Foursquare will now be check-ins with photos (rather than just check-ins)" the press release explains, hinting at a close partnership with Instagram.
Why's Foursquare doing this? It's a clever move, but has been so long coming that in some ways feels like it mirrors Twitter's slow pace of releasing new facilities--a kind of "treat 'em mean, keep 'em keen" business model that continually re-invigorates the core facility and bumps audience interest back up again. It's also another step away from the simplest check-in part of Foursquare's system--checking in is an act that, in itself, is pretty meaningless, and it's only the ancillary parts of Foursquare's system that makes it worthwhile: That's why Foursquare made moves to attract big-name  ad partners, with genuine offers to make to its users--it's trying to move beyond the simple "check-in." The addition of commenting and photos can also be seen as a rival, in some ways, to Facebook  Places--Facebook's system does integrate with Foursquare, but by setting itself apart Foursquare is trying to encourage some of the social interactivity that might otherwise happen on Facebook inside the Foursquare app. This increases user buy-in, because it adds to the user experience (possibly leveraging some Facebook fatigue,) and keeps Foursquarers on the system for longer, so they can get maximum exposure to ads .
To read more news on this, and similar stuff, keep up with my updates by following me, Kit Eaton , on Twitter.