I am pretty sure you can write a review in Steam as long as you have a Steam account. You don't have to have bought the product under review through Steam - unless they've changed things since last time I wrote a review... which was probably over 3 years ago.
I was talking someone else through the review process recently - I think you need the steam key now, plus at least x hours racked up playing it. Makes sense I suppose, you shouldnt be able to review a game unless you played it. The more reviews the better - so if people want to see these games succeed now is your time to help out and reccomend the game.