Thanks for the welcome!
Ignoring topics as well as following them has some appeal, but might just end up moving the problem, rather than solving it. For example, let's say you got fed up with seeing Nikon posts in the Camera Equipment topic, so you ignore the 'Nikon' topic. Only then to find out that you are missing a whole lot of great Landscape Photography tips under that topic because people were also tagging Nikon there, even though their camera model wasn't really relevant to what they were posting.
I think educating people as to how to properly tag posts, and when (or when not to) create a new category is really important. Giving users the ability to flag mislabeled posts might work, but will create more work for mods, but will somewhat distribute the work to the broader community. Mods only need to act if a certain threshold of complaints is reached. Maybe this can be automated.
Another method (probably difficult to implement from an engineering standpoint) would be to let users tailor how they follow a topic by ignoring keywords. This might let someone ignore mentions of 'Nikon' in a Camera Equipment category, while allowing them in a Landscape Photography category.
Categorization is certainly a big puzzle. You don't want to be too broad, because as you say broad topics like 'Photography' become overwhelming. You don't want to be too narrow or niche either, because there won't be enough followers. And when there are too many topics, it becomes hard to browse through them to find what you want. For example, I noticed there is a category for Hurricanes, and another for a specific hurricane, This raises a bunch of questions (what is the long term usefulness of the specific hurricane topic? Does each hurricane deserve its own topic? Or is this something to be handled at the conversation level) but also suggests that maybe nesting of topics is needed. At the moment there are some broad categories, and underneath those are topics. But shouldn't a topic for a specific hurricane be even one more level below the 'Hurricanes' topic? Photography should probably be a category, with Camera Equipment a topic, and Nikon a sub-topic. This would clean up the experience of browing topics to find ones that are of interest to you, which is only going to get messier over time. I'm not sure if that's enough of a benefit to justify the work involved, but I suppose followers of the parent topic would automatically be enrolled in the niche topics, which might clean up their personal list of topics, too.
And how to you deal with that over time? I happen to have a big interest in the history of the Ancient Near East, for example. There isn't currently a topic for that, but I can create one. Do I create 'Ancient History' as a topic, knowing it might eventually be overwhelmed by posts on Greece and Rome? Do I go straight for 'The Ancient Near East', even though I do have some interest in Greece and Rome as well. Will topics like 'photography' or 'computing' have the ability to be split over time as their activity grows? Will users later have be able to move their old conversations when a new sub-topic is created? And are old conversations even relevant? Are conversations meant to stick around for future reference, or are they meant to be ephemeral? This is probably it's own topic.
Anyway, some of this is just food for thought. You'll go mad trying to solve every edge case that crops up, obviously. Heck, you'll probably go mad trying solve the core cases.