Important to make distinctions between critics, user scores, and sales.
People will say something is overrated because critics or the community like it and they don't, or because it sells well despite their negative feelings.
People will say something is underrated because critics or the community dislike it and they do, or because it sold poorly despite their positive feelings.
In the case of Okami, many felt it rivalled other big Action\Adventure contenders like Zelda, yet Clover was shut not long after it was released and for awhile it seemed it might be abandoned. In any case it certainly isn't the sort of breakthrough success that gets non-gamers playing games.
In the case of Call of Duty, it continually sells better than ANY other game and scores quite well with critics. While I personally feel its a quality example of an arcadey shooter, I certainly don't think its perpetually the BEST game on the shelves as its sales imply, nor always deserving of AAA or near AAA scores. It has been the sort of breathrough success that gets non-gamers playing games.