A stub is an article which still needs improvement or that which is not complete.
What if it looks complete to me?
Then don't put a stub tag on it, but don't remove one unless you know for sure it is complete.
What would make it not complete?
For item pages: missing source info (where does it come from), missing ingredient/reagent info (what is it used for), missing recipe info (what does it take to make), who sells it, missing tooltip info, patch/hotfix change info, etc.
For formula/game mechanic pages: missing formulas, missing data tables, missing citations, etc.
For game term pages: missing alternative term names, context about where term is used, etc.
For guild pages: what faction, what realm, leaders/officers, guild website link, etc.
For mob/NPC pages: missing fight strategy, missing items sold, vendor type info, missing infobox info, patch/hotfix change info, etc.
For lore pages: missing citations or reference info is the main thing.
For player character pages: what faction, class, race, guild, realm/server, etc.
For realm/server pages: what region (US/EU), what timezones, what type (Normal, RP, PvP, RP-PvP), etc.
In general: categories, disambig links to pages with similar names, external links, etc.