If you have to do X, Y and Z in n amount of time and you fail then a quick chat about why failed is very useful whilst the issues are fresh in peoples minds.
If you have to wait a few months for your next review then no one really remembers the intricacies of the event, so you aren't really able to flesh out what went wrong or what can be improved.
The friendliness part is vital too. Reviews can easily turn too formal and intimidating, putting the employee on the defensive. Most employees don't want to under performs so don't treat them like they are being accused of something bad.
And I cannot under estimate the fairness part. Only review based on established criteria. Be objective, not subjective. If you let other employees offer feedback then force them to provide evidence and don't just accept their opinion as provided. I was once told by manager that he was family friendly and he was flexible about arrival times, so long as I put the hours in and got the work done. When I took advantage of this for just one week I was given a beating in my review for poor punctuality. When I challenged my manager about his claim to be Okay with this his response was "Do you have that in writing?". I resigned a week later. They begged me stay I refused.