It differs from GPL in that it is "viral" also when served through the web, so it could mean that your whole web application must either be AGPL, or you must buy an exception from the developer.
AGPL is a new license, and so there are few examples of how it affects things in practice. Many developers (especially ones building open source under less-strict license) prefer to keep their distance.
Not to mention that there is no way to know how a viral license even works when dealing with a non-linked language like Javascript or HTML.
The goal of the AGPL was to keep people from modifying GPL programs that were essential on the backend, but then keep their modifications hidden. With the normal GPL, if you don't distribute a binary, you don't have to release your code. With the AGPL, the goal was to force people to also release backend code. But code licenses start to get very shaky when we aren't talking about binary-linked modules.
