The one thing I'd really like to talk about is the support. One feature that I wanted was missing, and it was promptly added. I also asked about a minor change and was promptly told how to do it. All questions, even off-topic ones about Joomla configuration, were quickly answered in detail. Just great.
Several reasons: The others were way too powerful and complicated for my needs. But beyond that, they didn't look as good, and among the three that I tested I had the least problems in getting Discussions installed and running the way I wanted it to.
Even though it is quite basic, Discussions has several features that are beyond what I need, but it's easy to disable the extra facilities. And I'm especially pleased with the attractive and simple visual style.
So why only four stars? Well, the one thing that was problematic for me, and that cost me many hours of customization, is that Discussions is difficult to customize so it fits into a website style of light-colored text on a dark background. Discussions includes one CSS template, but it does not use it consistently, so much of the formatting is a mixture of the Discussions template and the basic template for the Joomla site. This can be very problematic - the initial installation on my site was showing light gray text on an off-white background! Almost unreadable. I even had to replace all of the smileys because the ones supplied with Discussions do not have proper transparent backgrounds, and looked ragged and ugly on a dark background.
If your Joomla site uses dark text on a light background then you won't have this problem, and Discussions is a top-notch five-star product.
I can only hope, and encourage the author, to make a future version of Discussions that includes two templates, one for dark text on light background and one for light text on dark background, and that he uses these templates consistently, and tests Discussions on a Joomla site that is based on a light text on dark background template style.
I'd also like to encourage the author to add a Donate button to his website - that's how much I like his product, despite the customization problems I had.