I've had this question too until I realized that no one really uses the Ideas section to submit any complaints, so at this point it just looks like a remnant from the past, something that tom has forgotten about, perhaps.
If "the rating system is broken" as the example mentions, then it's safe to assume that the average user would first think of reporting a bug rather than making a "complaint" about it, since that's what's encouraged around here anyway, whereas the "complaints" option is completely forgotten, and not even promoted by any staff member as an effective means to complain about/report something around here.
Bug examples: - Ratings system isn't working properly (i.e., rating not going through, rating showing the wrong value, etc.) - Posting tools not working properly (i.e., hitting the Reply button does nothing, hitting the Edit button wipes the original message entirely, etc.) - Certain features are broken (i.e., screenshots not loading or loading wrong images when selected, uploads not going through, blank pages, etc.)
Complaint examples: - Rules not making sense/redundant (i.e., "this rule doesn't make any sense and should be defined a little better", "this rule is a bit too restrictive and should be removed/adjusted", etc.) - Site features being frequently abused by others* (i.e., "this stamp keeps causing all sorts of problems between other users", "this unlock is constantly used by others to submit spam content on the site", etc.) - Site efficiency problems (i.e., "the site's taking way too long to load", "the posting tools should be expanded/simplified to make them easier to use", etc.)
*: Non-specific, not listing any names or calling out specific users.
Anything being reported about a specific user or group of users should be addressed by submitting a support ticket instead.