One of the growing trends in blogging is using a service to supply comment functionality, rather than using native comments. There are advantages and drawbacks to using an external system for comments, and each service provides both interesting features and potential workarounds for issues that are endemic to the idea.
Via that snippet, the service supplies all of the comments previously submitted to that post, a form for submitting new comments, and many potential optional controls that constitute some of the value-add of using the comment service.
There are without doubt some concrete advantages to using a service to manage a site's comments.
Comment services typically provide some degree of spam protection. Since your comments must all pas through the service's system for storage, the service will typically introduce spam-prevention into that workflow. The submitted comments are passed through a typically proprietary set of rules for determining if a comment is spam or ham, and the comment is added to the approved list of comments only if it passes those rules. This often obviates any additional spam prevention mechanism related to submitting comments.
One of the more powerful aspects of using a comment service is that they aggregate comments per user. For example, if you leave comments on both Alice and Bob's sites, and they both use the same comment service, you can see a list of all of those comments on your profile page on that service. This allows you to easily aggregate your own comments across the web and potentially re-publish that data even on your own site. You can also use this information to follow commenters on the web to other sites on which they comment. Typically, the comment service displays a link to a user's commenting profile within their standard output to a blog.
Comments services can also provide inline features on comments that are useful. For example, some services offer individual comment ratings so that visitors can vote comments up or down based on their value. Some services provide threaded comments, which might not be part of what the standard blogging package provides.
Comment services can provide easier methods to reply to comments that are submitted. For example, if you register with a comment service, it may be able to email you when you receive a new comment. Depending on the service's capabilities, you may be able to reply to that email and effectively add a reply to that comment on your blog.
Such advanced features aren't without their drawbacks. Many of the available services offer ways to compensate for these drawbacks. Check with each solution before assuming there isn't a way to mitigate the problem.
On some comment services your data is not portable. This problem is sometimes bi-directional. Some services will not import your existing comment content, so if you have a bulk of comments on your site already, you will lose them if you switch to a service. Some services offer no way of exporting comment data that they have accrued, so if you decide you want to revert to the native comment features of your blog software or switch to another service altogether, you may find that your comments are locked inside that service.
Comment services do not share aggregated comment data. If Alice's and Bob's blogs use different services to manage their comments, and you leave a comments on both of their blogs, you will not see comments from the other blog aggregated in your commenter profile. You will need to switch to the other service's profile to see comments left via that system. If half of your friends use one system, and half use the other, there is no single way to monitor all of the comments, defeating one of the major advantages of using a comment service.
Lack of sharing aggregate data is a serious problem, and there is no service that has overcome it. There is no proposed integration standard to make it work.
This is not a comprehensive list of services, but a basic representation of the marketplace. There were no fewer than six vendors of comment services represented at Blog World Expo this year, and more have surfaced in the few weeks since then.
js-kit is slowly integrating itself into larger and larger players. CoComment, one of the original comment services, now uses js-kit. js-kit was also derived from the original Haloscan, a service geared to providing flexible commenting on sites that did not have commenting ability. One of js-kit's interesting features is providing YouTube video integration as part of commenting capabilities.
IntenseDebate was recently purchased for use within WordPress.com for comments, suddenly gaining marketshare in the millions of WordPress.com users. It's features include the basics of rating, threading, and replying to comments via email.
TypePad Connect was recently rolled out by Six Apart to enter the comment service marketplace. TypePad Connect provides similar features to the other services, and adds OpenID integration for user verification.
Disqus includes basic rating features and threading, as well as inline popup profiles on blogs in which it is embedded. Due to its early arrival on the scene and development explicitly as a comment service, Disqus enjoys a large market share.
SezWho is another service that offers basic commenting and rating features. SezWho provides an administrative interface that reveals the number of visitors their service is driving to your site by means of aggregated commenting.