A solid site architecture is essential to any site hoping to compete in the online world. Your site architecture is, firstly, essential to your users. A logical arrangement, accompanied by on-page elements such as breadcrumbs, Johnson boxes, obvious navigation paths and page headings and sub-headings will ensure that users receive a clear signal that their current page is where they need to be.
Secondly, a valid, logical site architecture is essential from the point of view of the search engines crawling your site. A hierarchial arrangement of pages implicity provides a 'weight' to each page, depending on its depth. E.g. your homepage is at depth 0 so it's naturally more important than a leaf page at depth 5. You should bear this in mind when thinking about your site structure.
Consider your landing pages for both short-tail and long-tail terms. Pages targeting short-tail terms should be closer to the top of the hierarchy and there should be fewer of them than pages which target the long-tail. This might seem obvious but we often see sites where the ratio of category pages to product pages is 1:1 or even 2:1. How can that be possible? Well, one side-effect of multi-categorisation is that a product, and its corresponding page, can have 2 parent pages. If each of those parent URLs are crawlable by the search engines, the structure implies thin content, a serious no-no in a post-Panda world.
Several years ago, having as many category pages as possible was the de-facto standard. The theory being to provide as many avenues to product pages as possible. That theory still holds true but only to a point. Much thought and technical expertise needs to be invested in how to differentiate those category pages from each other. If you don't have content to justify the creation of multiple category dimensions, then you must consolidate your creative efforts and your site architecture around relatively few categories.
Ensure that your category and product pages are heavily interwoven and ensure that they themselves sporadically incorporate other elements of your overall website, e.g. blog, news &c, as well as reputable third-party sites of interest to your users. Those other non-core sections should heavily reference both individual product pages and category pages. This is a parallel of the 'deep-linking' concept central to modern SEO.
The principal point that needs to be carried through is that your architecture must reflect your ability to create unique and valuable content at every page level. There is a balance to be struck. If you can't see yourself crafting content for every proposed category, consolidate those categories and increase the ratio of product pages to category pages. If you're confident of your ability to 'fill' your website, always start conservatively and add categories as required. Adding them at the outset and thus creating an empty shell of a site will not be viewed positively either by your users or the search engines.
Recently, we were asked to offer some advice to an estate agent in the south of France. They have a wide range of properties for sale however many are either concentrated by area or by type. Clearly they have other properties aside from those in the majority but naturally the product:category ratio will be quite low. It was important to make those category URLs SEO friendly but to concentrate the 'juice' to the category pages which contained most of the products. It means that users arriving from search engines get what they're looking for quicker, which means they're more likely to convert. There are 2 predominant side effects then with regard to SEO. Firstly, the heavily linked category pages will be given more weight in the overall site structure and secondly, the bounce rate should decrease as a result of the re-prioritisation to product heavy categories.