Hi,
I think we have two issues here
1. Duplicate content. This is something that can be solved by adding unique content on the category/tag pages. So, if each post, article is published into one, and only one category (it has only one URL) and the categories have extra unique content (text) on them, I think we avoid the duplicate content issue. The same is for tags - adding content reduces the risk of having duplicate content. Some tags may overlap some categories, but this is why adding unique text would solve the duplicate problem. Personally, I use 3-5 main categories (main classes of posts), and another 3-5 tags (some subclasses) to offer a better, narrower, in depth grouping.
2. How good is this page as a landing page? Well, I think sometimes it is better to let categories/tags be indexed. Let's say we have a category/tag page, with many products on it; in my opinion, this category/tag page would be a better landing page than a single product page, for instance, with a better conversion rate. I alway ask myself - would this page be a good landing page (be it a post or a category/tag page)? Do I really want it to be a landing page? If the answer is no, well noindex.
Correct me if I'm wrong.