The most fundamental problem is the risk involved in making a decision. When you hire the wrong person it hurts badly. It's painful, stressful, and expensive -- usually for both parties.
One way I've found to mitigate this is by having potential employees work on a contract job first.
The trend right now is for job sites to employ programming tests and the like, but I don't think these answer most of the hard questions you have about a potential employee.
A hybrid Elance/Monster might be just the ticket though. Employers post real jobs, like "Port our Oracle stored procedures to Postgres" or "Rewrite this Ruby server daemon to support XYZ", then they work together with candidates on the project, while paying them. Employers would make full-time offers to people who do well.
Employers might pay three candidates to do the same job and then pick which one did the best. Still far cheaper than a recruiter.
Candidates that don't look good on paper might even volunteer to do work for free/cheap just to prove they have what it takes. I know I would have before I had job experience.
As a developer who loves programming, but looks very bad 'on paper', I would totally be down with this idea. Sometimes when I'm interviewing at a company, I just want to tell them, m "Lets skip all this formality crap, and hire me for a week. If you don't think I'm good enough after one week, then you can fire me without pay".
Elance does divulge where these people live- you can restrict your hiring/contracting patterns accordingly.
Similarly, if you are up for reloing candidates, you're not restricted to a local hiring area.
Also, you can make your wants and needs transparent up front. If a freelancer doesn't like the idea of working in your office for a salary, they're free to be honest about it before you hire them for the contract.
I think it's kinda silly to imagine a site that's a crippled version of what's already out there when the existing solutions are more than capable of sufficing in this limited case.
Logistics. More time is spent making posts, culling resumes / cover letters, scheduling phone interviews, coordinating in-office meetings with the team, and following up than actual searching. Even with an admin assistant, it's a huge burden on me.