Comment extensions are a popular feature to add to Joomla websites, they are a lot of choices, all with slightly different features.
Have you ever been to one of those sites where all the comments are filled with spam? It's like a really messy house, it shows me that the owners don't take pride in their website and they'll lose credibility.
Another problem I'll see alot is the flamefest. Back and forth, commenters, and sometimes the author, will spiral a discussion into name calling and rudeness. This is usually a sign of a lack of an obvious comment policy and active moderation. Always make sure your policy is clear and you hold comments to it.
The flip side of online diatribes is another problem... the tumbleweed blog. Most often seen on blogs embedded in company websites. None of the blog posts have any comments, and no-one seems to be interested in what you have to say. Not a good indicator of your products or services.
Did you start out strong and then stop posting? This is a sure-fire way to get your visitors to stop coming back. If you went to all that effort to get them coming to your site with your useful information/opinions, don't drop the ball and make them leave.
In the same way, when you are considering comments for your blog, you need to make sure you answer some more important questions before you lose sight of the big picture and get caught up in what comment extension you'll be using.
1. Why do you want comments?
What are the goals of your website. Have you you nailed those goals, and would visitor comments actually help them?
2. Will your blog be interesting enough for people to comment on?
Make sure you have 6-12 riveting posts ready to roll before you even create that menu item to a blog. Look at what others in your industry are talking about, and see if you can add to that discussion. Its also worth developing a specific strategy to try and get those all important first comments - "maybe people will comment" is not a plan!
3. Will you have time to keep your blog clean?
Writing the content is only half the story. Make sure you have a plan in place to maintain your blog. Spam needs to be removed, spammers banned and comments moderated. Make sure you clearly show your post policy. "Don't write anything your mum wouldn't like" is my favorite!
4. Do you know how to spread the word?
Your blog isn't Kevin Costner's Field of Dreams. If you build it they won't come. You need to actively promote your blog and individual posts. You should have already figured out what community your posts will be read by. Go into that community and tell them about what super duper interesting stuff you have to read.
Got all these 4 things sorted out? OK, NOW go out a pick which comment extension you'll use on your website.
Me? I like Disqus. Its an online service and needs a plugin to work on Joomla from Joomlaworks. Its got strong hooks to Twitter and Facebook (see point 4 above) and has a nice slick interface.