Usually when I thank someone using the Thanks extension, I end up also leaving a message on the user talk page explaining exactly why I found their edit helpful. The thanking feature should provide an opportunity to accompany thanks with a brief comment (e.g. 255 characters or less, so it could fit in log_comment). That will save users steps and help aggregate information on why users find the edits that they're thanking people for helpful.
Version: unspecified
Severity: enhancement