Last updated February 18, 2015.

The following general-purpose tags are not specific to any particular project issue:

Accessibility
Affects the ability of people with disabilities or special needs (such as blindness or color-blindness) to use Drupal.
CSS
Involves the content or handling of Cascading Style Sheets.
Docs infrastructure
Infrastructure/code improvements related to documentation. Note that the Core Documentation team uses this tag, not the next one.
Documentation
Primarily changes documentation, not code. For Drupal core issues, select the Documentation component instead of using this tag. In general, component selection is preferred over tag selection.
Drupal.org upgrade
Involves a recent or proposed upgrade to the drupal.org website.
DrupalWTF
Worse Than Failure. Approximates the unpleasant remark made by drupal developers when they first encounter a particular (mis)feature.
DX (Developer Experience)
Enhances developer experience.
Favorite-of-Dries
Please do not use this tag unless your name is Dries Buytaert.
HTML5
Implements and supports the use of HTML5.
D8MI
(Drupal 8 Multilingual Initiative) is the tag used by the multilingual initiative to mark core issues (and some contributed module issues). For versions other than Drupal 8, use the i18n (Internationalization) tag on issues which involve or affect multilingual / multinational support. That is preferred over Translation.
Infrastructure
Issues relating to drupal.org infrastructure that are posted in other queues such as Project or Project issue tracking.
Javascript
Affects the content, performance, or handling of Javascript.
jQuery
Affects the version, handling, or usage of the jQuery javascript library.
mobile
Issue related to Drupal on mobile devices.
Novice
Would make a good project for someone who is new to the Drupal contribution process. Preferred over Newbie.
PHP 5.3
Particularly affects sites running on PHP version 5.3.0 or later.
PostgreSQL
Particularly affects sites running on the PostgreSQL database.
Quick fix
Very small and simple change. Preferred over Quickfix.
Usability
Makes Drupal easier to use. Preferred over UX, D7UX, etc.
VDC
Related to the Views in Drupal Core initiative.
Views
Involves, uses, or integrates with views. In Drupal 8 core, use the VDC tag instead.
WYSIWYG
Involves an in-browser What-You-See-Is-What-You-Get content editor.