Trilarion: First hand experience is much more valuable then just seeing some screenshots and reading what others write about something. You can have your opinion, but I wouldn't trust it much unless you tried the game before. I guess that is the reason for the limitation on the review page. It doesn't mean you cannot have an opinion, it just means that we trust you much more in this regard if you have first hand experience. I think that's reasonable.
I'd say that it depends on the game aspect that is written about in the review. To make a competent judgement about story or party companions one should complete a game (it is not uncommon for me to change opinion about companions in a game like DA, PoE, etc., short description on the game site is not enough), as it is something that this player has not seen yet. To make a competent judgement about game features that were already seen and experienced in many other games one doesn't need to start that particular game. For example, I don't need another first hand experience to say that checkpoint save system in a CRPG is bad game design, I don't need another first hand experience to say that overly simplified dialogue system (yes, no, sarcasm) in a story heavy game is bad game design. Experiencing it once more will change nothing.
Speaking of trust, having game in a library says nothing about game being played. Having a number of "hours played" says nothing about game being actually played (and not left in the background for the sake of review, gamecards, etc.). Having achievements says nothing about game being actually played (there are special programs that will give you achievements in Steam, no idea about Galaxy).