I can understand the reasoning behind the lower scores. FF13 is the most linear Final Fantasy in history. And by linear I'm talking about gameplay, not plot. As GameSpot mentioned, exploration is pretty much navigating straight through narrow corridors, not much to do outside of the main quest, etc. These things are bound to frustrate a lot of FF veterans who enjoyed the non-linear aspects of FF7-FFXII. And according to GS, character skill development is also pretty linear too. Apparently there aren't too many branches on that skill board or whatever they call it. GameSpot was very fair in their assessment of FF13, they just docked it points for the extremely linear gameplay. So they gave it a 8.5 (not much higher than the metacritic average).
Critic scores aren't that important anyway. What I do find irritating is when critics do a poor job of actually writing the review. Now that I have a rental subscription, I don't have to worry so much about this though because I can just try out games at little cost to myself.