I don't know if GameSpot's scoring system has changed but many years ago I remember that anything in the 6 range meant that the game is strictly for fans of the genre and those who are only casual players of the genre or barely play those games should give it a rental first. They would have little explanations like that for their scoring system. If you play a lot of jrpgs, chances are your standards for a jrpg are going to be lower than the standards of the mainstream gamer that only wants to play the cream of the crop jrpgs (in the critics' minds, stuff like Final Fantasy. I strongly disagree that FF is the cream of the crop but the hype of these games is so strong that it convinces people that these games are the best jrpgs).
So unless you feel WKC is under-rated with respect to other jrpgs that have been rated by GameSpot, then I don't see why the 6 is so bad. GameSpot went against this rule by giving Demon's Souls a high mark. But it could be that the particular reviewer who reviewed that game liked those kinds of games (dungeon crawlers as opposed to traditional jrpgs.)
Valkyria Chronicles was rated highly (8.5 or so?) but it could just be that reviewers prefer SRPG gameplay to jrpg gameplay. Which is perfectly understandable. I haven't played VC but generally, srpg battles make every skirmish (except for the odd random battle like in FFT or whatever) count. Every fight is like a boss battle except the boss has a big posse in these games. They don't have boring, tedious, throwaway battles like they do in traditional jrpgs that do nothing but bore you, create tedium and add nothing to the game except increase your stats.