Magento seems to have lots of email templates to send a variety of different messages based on user activities: registration, password recovery, order, shipment, etc.
Where can I find all the email templates so I can customize them ALL and never miss one of them to avoid looking bad when the user sees a poorly worded default template?
Are they all located in one location / directory?
app/locale/en_US/template/email/ is the default place/locale. You should not customize them in the filesystem, or if you do, plan on protecting & merging when you upgrade.
You can customize them (albeit somewhat wonkily) via System > Transactional Emails. In this area you select the base file for your locale and alter the content, which is then stored in the database.
Here is a free plugin that lets your override the email templates: https://www.yireo.com/software/magento-extensions/email-override
This is where you place the overrides:
app/design/frontend/{PACKAGE}/{THEME}/locale/{LOCALE}/template/email
So if I was overriding this file:
app/locale/en_US/template/email/account_new.html
and if my package name is "package" and my theme is "default", I would place my override file here:
app/design/frontend/package/default/locale/en_US/template/email/account_new.html
