Here's my consideration on whether to create a category or not. The answer to all would ideally be "yes."
- Would a post/page (either one-time or regularly updated) be insufficient to talk about this topic?
- Does the term receive significant search volume?
- Can I cover the topic well enough and regularly enough to satisfy users and search engines?
- Does this category have value to a user? Would a user think, "I want to see all the posts about ___"
- Does the category avoid heavy overlap with another category?
This mostly apply to tags as well, but I tend to use tags to talk about themes (e.g. an ongoing story in the news), brands, etc.
Alright, so those are the abstract rules as far as I can think about them. Let's look at some examples.
- iTunes iPhone apps: Seems like a good one. You can't cover it in a post, there's plenty of search volume around iPhone apps, there's plenty to talk about, users would want to see only iPhone apps. I do think, though, that you have a ton of overlap. There's a category for "iPhone app reviews" and another for the type of app it is.
- Divorce apps: Kill it. Of the 3 posts in there, only 1 appears even somewhat relevant. Search volume is negligible, and you're not really helping anyone who navigates here.
- Cartoon Network: Should probably be a tag, but only if a user would find it useful.
- Paid: Not sure that people are itching to find paid apps - and they're likely to find the Play/iTunes store anyway. Actually more people look for "best paid apps" than for "paid apps," This is one that should be covered by a post rather than a category.
- Top rate: this one's tricky because it's tempting to think there's plenty of people searching for "best apps" and "top apps." Here's what I'd do instead: kill the category; post a monthly list of the best apps overall, and update it. Then get more specific: the "10 best tower defense games for android" and such. I'd kill it because every single post in the category would overlap.
I also noticed that your categories are not optimized. The "free" category, for example, is simply titled "Free." You'd want something more like, "Free Apps for iPhones and Android Phones." (just an example) There are a number of plugins (including one by Yoast) that allow you to set custom category title tags, which I'd highly recommend.
I can't go through each of these for a Q&A, but my structure would look a bit like this:
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Device
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App type (includes reviews, news, commentary, etc.)
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News
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Industry news (because all new apps go under app type)
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Device news
When you're deciding on whether a type of app deserves its own category it's hard to know whether a user would find it useful, and you can make the argument for really obscure things that 1/1000 people would actually click on. Instead, start with search volume, and aim for simplicity over comprehensiveness.
If you do change category hierarchy, make sure to find category pages where you're receiving search volume. It's not likely, but you don't want to be moving any structure around if it's working well for you.
You're asking a big question, so I apologize for the long answer. Hopefully this gets you started. Let me know if you have a few specific questions 