While there is inappropriate content on here, openly displayed, most of it is in this little thing called Guilds.
Firstly, it is against Gaia rules and ToS to discuss mature content or attempt to use the site as a dating network, and there is an age minimum of thirteen now, I don't know if that was always there. Not that children always follow such rules, but parents can find these types of things out easily. So the site is designed for a slightly older age group, as opposed to sites such as Club Penguin or Littlest Pet Shop (sites my younger siblings play on.)
Now, the main forums are moderated by people that Gaia has deemed responsible through their actions. Not to mention, general users can report other users for breaking Gaia Rules and ToS.
The reasons most of the inappropriate content is in Guilds is because Guilds are user-moderated. Usually, you can tell what type of content is in a guild just by its main page. The larger guilds are harder to self-moderate- don't expect to find good moderation there. When a guild is reported, then moderators step in, but until the situation is bad enough that this needs to happen, people are going to continue with their actions.
Gaia tries to maintain a pleasant but open to opinion environment, but with over a hundred thousand people on each day, that can be difficult.
As for my little 'Positive messages' icon I have clicked, that's due to the fact that there ARE good people on there that support things like cancer awareness, and places where you can find a nonjudging voice to listen to a problem.
There are many, many opinions in this online community- and people are a good deal less shy about sharing them online. So, as with many sites, there will be ideas you maybe don't want introduced to your child- but that's everywhere, and other than not letting them into the world, there's not a thing you can do about it.