I don't have any personal experiences with Free, Arena or Tabletop. My opinions of those sections are that Arena is comatose, Tabletop probably has a good case for existing, and that it is embarrassing to be writing on the same website that hosts the Free section.
The Casual section is RPG. It is the steak and potatoes of the Guild, and where more than half of all user activity occurs. Obviously, this is both a blessing and a curse. It means that the section is very active and that new roleplays pop up daily, but it also means that the quality of writing here is very hit or miss and that all of RPG's unfortunate peculiarities (e.g. certain genres being very overdone) show up in Casual in their masses. I have written in roleplays in the Casual section and enjoyed them, but I also know that most of the sorts of threads posted here aren't worth it for me to click on.
Advanced is great. A lot of interesting roleplays crafted by people who both want to tell a good story and know how to do so. I have no complaints. That long-running meme about Advanced RPers being pretentious snobs hasn't had any real basis for years now. More of you people who usually roleplay here should start posting in the Nation RP section. Speaking of...
The Nation RP section is my own personal favourite section of RPG, and the one I post in most often by a wide margin. More worldbuilding happens in each individual sheet posted here than in some entire roleplays in Casual. The stories that can get told here are both of a grand scale and almost always involve a heavy dose of political intrigue--a favourite of mine. The main drawback is the relatively low level of activity, which means that a lot of cool ideas never really get off the ground. I will also eternally take credit for the Nation RP section existing; I swear that I gave the idea to Mahz and he added it in that same day.