Right off the bat the site has a huge problem. Any data gathered will have a pretty strong selection bias - people who have positive interactions with the police are unlikely to visit the site, so the only people to write reviews will be the ones with complaints. Statisticians refer to this as the "whiny bitch bias." The whole site could very easily turn into a big cop-hating bitch fest. Anyone who's predisposed to disliking cops, will upon visiting the site be presented with a litany of evidence that all police everywhere are arrogant dicks. The truth - that no more than 60% of the nation's police officers are arrogant dicks - will be lost on them.
On a positive note, rich new veins of pig/bacon/pork related humor may be discovered and exploited within the site's forums.
Any attempts to compare police officers or rank them will also be flawed (Try to imagine this site not having a top 10 list of the worst cops in the union.) Police officers will have different reviews based on their different duties and the community they work in. An officer who works traffic duty is going to be taking a lot more flak than that guy they send out to talk at school assemblies. And a straight laced cop might unfairly garner negative reviews if he was teamed up with a burnt out cop who plays by his own rules.
Also, how will the site handle cops who are identical twins?
From the police's point of view, the worst case scenario here is sadly very plausible: Certain individual cops will be singled out on the site, unfairly or not, and will then be harassed by the site's most ire-some members. Maybe that harassment will be of the legal variety (formal complaints,) but what if it's the distinctly illegal sort (poo in the mailbox?) Clearly unacceptable, but it also hasn't happened yet. Can a site be shut down because it may one day be the root cause of a hazmat team gingerly holding a mailbox in a pair of enormous tongs? Probably not, I guess.
Most importantly, and I say this in my full legal authority as a Cracked blogger, I'm pretty sure there's nothing actually illegal about this concept at all. Police officers are public figures, their names and badge numbers are public information, and complaints about them are already handled in the open. Any attempts to make police officers more anonymous or to make police-work less transparent should be strongly resisted, lest it provoke a wave of bloggers flooding the Internet with clumsy references to 1984, or worse, Judge Dredd.
In short, I think the site could so either good or harm, but probably not much of either. Consequently, I don't think it should be illegal, but I also won't be checking it out. Kind of like fecalphilia, or American Idol.