Stumbleupon should allow readers to add new interests.
I just stumbled a post that has to do with prosperity and the law of attraction. There was not ONE interest in SU that described this article! I ended up listing it under "health" because I couldn't figure out anything else. Pitiful!
And this is not a one-time problem. I encounter it frequently when stumbling people's stuff.
So I know there is a billion categories already and we can't have the perfect category name for every page out there but I still wanted to suggest this. There are a ton of internet comics out there and they don't really fit into "Cartoon" or "Comic Books". Most people's perception of comic books is something superhero/action related that is not humorous, and Cartoons are something you watch not read, but Comics people see as being the funny comic strips you see in newspapers (back in the day when people actually read them. They do still exist, right?). I figure you already have a billion categories so why not just make it a billion and one.
Please add Leadership as an interest category. Currently, people have to put articles under Management/HR or Business but leadership is extremely popular on its own (actually more popular in Google search than Management or HR. It is a much bigger interest area than just business or management or HR.
I think that there are people out there (like myself) who are either interested in, or collect historical weaponry expanding anywhere from smaller knives and daggers to swords, maces, axes, clubs, etc. Being able to stumble through some of them would be exciting to me and I'm sure others would agree. It could even expand into catapults and larger things like that. I also know that there are plenty of articles written on the impact of certain pieces of weaponry or armor (e.g. Greek Hoplon) and their historical significance. It seems like a category with a lot of potential, that could be both for fun and educational.
You should create a "My Little Pony: Friendship is Magic" interest. You'd be surprised how large it is on the internet. It's not just for little girls anymore. Many adults watch it. Google "mlp: fim" if you don't believe how large the fandom is. I dare you. Bronies are everywhere.