I have been working on a project lately where I have been dealing with filtering and categorizing large amounts of data, specifically retail products. What surprises me is that companies who are in the business of helping people find a product, don't take the time to lay the proper ground work to make it easy for people to find what they are looking for.
Let take for example, Amazon.com, one of the largest internet retailers. Its their business to help people find products. But Women's T-shirts, with 25,000 items is very hard to navigate because it doesn't really have any subcategories. Off the top of my head, I would think you could create sub categories like, V-neck, Round Neck, Cotton, Cotton Poly Blend, Novelty, Casual, Retro and more.
It does us no good to give people all of the products/data in the world if there is no way to easily navigate that data. If you are in the business of helping people find something, take the time to do it right and make sure everything is properly categorized.