Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and openWorldHint. Beyond that, the description usefully discloses that data is credential-free public data, that upstream comment bodies are not exposed (important expectation-setting given the 'comments' name), and that the ranking window is roughly a week. It stops short of describing pagination or result ordering details.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.