Don't you think it's better if there are several persons asssigned to review one's account when they reached the number of activity required?
It's up them whether a certain account would rank up.
There are more than 5000 new registrations happening here every day now. A lot of resources would be required to review the accounts.
Even if you get enough people to do those reviews, it will be more biased and easier to cheat. Imagine if someone gets into an argument with some people who later will have to review his account. This will create a whole lot of new problems.