Off the top of my head (coffee hasn't happened yet), and with the two caveat that I'm not all that wedded to the names below, but more to the ideas/descriptions therein
order-workflow -- any question dealing with data and workflow of taking a placed order and getting it to a state where it's paid for, shipped, and off to the customer. Roughly anything under the sales menu.
payment-modules -- any question dealing with the creation of, integration of, and support of payment modules. The things that let you interact with money making systems (AuthorizeNET, PayPal, Stripe, etc.)
shipping-modules -- any question dealing with the creation, integration of, and support of shipping modules. (UPS, USPS, FedEX, etc.)
tax-rules -- any question dealing with Magento robust yet arcane systems for managing state-wide and product specific (weee!) tax systems
catalog-promotions -- any questions dealing with the setup of catalog (side wide) promotional rules
shoppingcart-promotions -- any question dealing with the setup of shopping cart promotions (coupons)
cms -- any question dealing with Magento's CMS and static block features, as well as questions related to integration with third party CMS systems
widgets -- any question dealing with Magento's CMS Content-Type/Widget features (widgets feel complicated enough to warrant their own tags)
reporting -- any questions dealing with Magento's internal reporting system, as well as questions about integrating with third party reporting systems
inventory-management -- any questions dealing with integrating Magento with inventory management systems, or attempting to use Magento itself as the source of truth for inventory management
magento2 -- any question dealing with Magento 2, which will be out a few quarters from now. For reals.
developer -- questions oriented toward someone attempting to extend Magneto's functionality, or features that require extending it
site-builder -- questions oriented towards someone attempting to use Magento (+ 3rd party extension functionality?) to build out their store/system
transactional-email -- questions related to the design, sending, and management of the many transactional emails sent out by magento
api-soap-xmlrpc -- questions related to using Magento's original SOAP/XML-RPC API
api-rest -- questions related to using Magento newer oAuth/REST API.