...There are about 100 categories. All incoming links would then be spread evenly over the most importent parts of the site. Would that be a problem? From a user point of view it makes a lot of sense.
IMO, it's a major problem, both for users and for SEO. Too much choice is no choice. It's inherently user-unfriendly. It also makes it essentially impossible to prioritize anything on the site.
You may see sites like Amazon showing lots of categories from home, but they have orders of magnitude more authority than you do... they are much larger sites... and they get a lot of inbound linking directly to category and product pages, so link juice distribution from home is not an overriding factor.
Additionally, within categories on the well-optimized major sites, the subcategory links become focused within categories. They are never global dropdowns.
Here are two, of our many discussions on the topic, that are worth reading....
While 100 categories isn't necessary a mega-site (depending on how you define your categories), many of the considerations discussed still apply.
Still presents confusion to the user... and Google increasingly doesn't like to see different navigation than what the user sees. It also misses the basic point, that tossing everything into a set of drop downs is simply a sloppy way of solving your navigation problem.