What about a site with better book categorizations?

The problem is in data entry! Someone would have to enter book details. To use Amazon and Goodreads' APIs you have to agree to conditions that basically prevent you from using their data on a site that would be in competition to them. I can't remember the details for Goodreads but for Amazon, you can use product details only to help sell the product. Which I suppose would be fine if you put a "buy from Amazon" button on the page...
However, you would still be dependent on users adding book metadata.
There are more than 100 000 romance books on goodreads. You can only browse up to 100 000 which is why they show that limit. Let's look at Amazon...
More than 10 000 kindle romance books have been added in the last 30 days. Most of them probably suck but still. Part of a system like this is wading through the thousands upon thousands of KU books available.
On fantastic fiction, there are 1800 new books published in the last 30 days.

If you have 100 users, each adding 1 book a day, it would take 2 years 8 months to add 100 000 books.
That's without the thousands of books being added each year.

So you would be facing quite the slog to add tags. Unless you could scrape tags from goodreads, but then you run into the terms and conditions again.

Every now and again I get a wild hair and start setting up a sql server database for all my books, but then my anal-retentiveness kicks in and I can't decide how to set up the tables. I feel that a database would be perfect if it can handle Jayne Ann Krentz, with her aliases, multi-author series, single author series, and story arcs. I'm still working on it My sister is a sql report writer maybe I should get her input. Anyway if you want to discuss it I'd love to have someone to brainstorm with.