Information that is diagnostically helpful to people more than just developers ( IT, sysadmins, etc.).

Any error which is fatal to the operation, but not the service or application (can't open a required file, missing data, etc.).

Any error that is forcing a shutdown of the service or application to prevent data loss (or further data loss).I reserve these only for the most heinous errors and situations where there is guaranteed to have been data corruption or loss.

Generally useful information to log (service start/stop, configuration assumptions, etc.).Info I want to always have available but usually don't care about under normal circumstances.

Only when I would be "tracing" the code and trying to find one part of a function specifically.

Anything that can potentially cause application oddities, but for which I am automatically recovering.(Such as switching from a primary to backup server, retrying an operation, missing secondary data, etc.).