The hard part is staying one step ahead of less-legitimate accounts. There's always somebody trying to cheat the system, and stopping these users often results in a tradeoff.
But a continuous, consistent pattern of one-shot negative reviews is also not particularly difficult for a competing business (or anyone with a grudge) to reproduce.
This is a hard problem, and I don't think the credit check-style identity verification questions suggested in the post are going to solve it. For one thing, I think an overwhelming number of people would find it creepy. They're trying to tell people about a bad moving company, not get a car loan.
Putting more of your "valuable" online identity at stake seems likely to help. I hate to say it, but "Log in using your Facebook account" suddenly gives Yelp a lot more data to exploit to decide just how much trust they should put in your reviews.
Putting in my credit card (or revealing any information I don't deem relevant to the company / form) is always an immediate deal breaker. For Yelp, a critical mass of reviews is needed, I'm not sure they can afford that.
As far as the user is concerned it's just a few questions and answers.
If the questions were about my credit history then this is a deal breaker.
There is no way a site like yelp can get me to authenticate by answering questions about my credit history and I suspect that the vast majority of people would agree.
Now, facebook credentials, that's a different story.
Just a few questions and answers that could hint at all kinds of things including relationship status, orientation, race, financial status, etc. Imagine if some stranger on the street, or even the cashier at a trendy retail store, asks you about some detail they have no business knowing. You would find it creepy. Now imagine it's some random web site (which is all Yelp is to most people).
I have a friend who is currently a target of such a vendetta. He got into an edit dispute with someone who was trying to censor stuff on a wiki. This other person responded by flooding listings of my friend's previous employer (which has been out of business for a few months) on nearly every review site out there with a continuous stream of utterly fabricated reviews. These reviews range from calling him a pedophile (easy enough to get deleted) to pretending to be a client who was screwed by his incompetence. I'm not sure there is any way to differentiate the latter kind of reviews from genuine negative experiences.