One of the things I love about Drupal is the taxonomy system. It's convenient and stable... except when it's not. I got 404s for the term URLs (both system and alias) and the cryptic message "Notice: Undefined index: href in menu_local_tasks() (row 1973 in path2site/includes/menu.inc).". Well, here's what worked.
Flags is a very useful module that allows users to "flag" nodes that meet some criteria (eg type etc). Administratively, it might be of interest to show the users who have flagged a specific node. Unfortunately, that's both tricky and slightly non-intuitive.
I assume that if you've build a relatively large site, you've stumbled on Nodequeue by now ( summary: it allows you to create lists of arbitrarily ordered nodes ). But clients always had a thing for imagining things that are normally tricky to do, and one of them is to create a block that has arbitrarily ordered content per section of the site, usually by taxonomy term.
You really aren't using Drupal right if you're not harnessing the power of taxonomy, and you haven't seen much of it until you understand views. The only limitation for me was that you have to streamline all terms to look alike, in order to fit in the same templating. Well, not anymore!
If you're using Drupal without Views... you're probably doing it wrong (sic). Views really are almost a the only way to have full control over any content listing, with minimal effort. You can even customize things on a per-field basis via template overrides. But how do you add particular classes to individual rows?