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If you watch the youtube video's of Marshawn at Cal, he is very good in the open field. I think he has two weaknesses as a runner. (1) He doesn't have an elite first step (think Curt Warner) and (2) he doesn't have elite vision (think Marcus Allen). Both of these weaknesses are amplified by poor offensive line play.
Marshawn can make people miss and he can run through a tackle, but he can't make three people miss or run through three tackles (usually). If the Seahawks start opening creases -- not holes, just creases -- where they are designed to be, Marshawn will be an asset.