your general idea is more correct now yes. in theory the static sites are to play around with the idea of having a site, and as a way to get to know and share about yourself in a persistent way to the other users, they aren't really suitable for much more than that. most have disk quotas (lots of other people using the same computer really limits how much space can reasonably be used per person) so file share is possible but usually not heavy things, for that there are better protocols (but the links can be shared easily enough).
if you look at the services the tildiverse site talks about, things like the wiki, zines or radio, etc from the outside it might just feel like a public resource, but inside it's something you can participate in directly, so you want to try running a set on radio you can talk to people there and get a time, if it's well received, etc, you can become a regular dj for the server. things like that.
i don't know of any that would let a user set something up like a textboard, but i don't know all of them and they all have different community cultures. more of them may be open to such a project as a community project, more likely than as a user one tho. i do know some of them have "premium tier" levels of membership that provide more access to things, such as cgi compatible webhosting, or even stuff like SIP trunks for PBX