I was about to say that, and then you said it first! :)
If you have one character whose brilliance (or lockpicking skills or hacking or angst or whatever) is Their Big Thing at which they surpass all others -- then suddenly, all too often, the other people around them get made into incompetents, not even also-rans, who exist to be wowed by the one guy's skills. When often, in canon, Watson (or any number of others) was perfectly smart and perfectly competent, even if he wasn't in Holmes's league in Holmes's chosen specialization.
Of course, canons sometimes do this to their own characters too. (The above example of Dean from Supernatural is one I saw nearly all my friendslist up in arms about; they felt that all characterization said that he should know that perfectly well, and he got dumbed down to make other characters look better even though it made no sense. I don't watch Supernatural, so I don't have a strong opinion on it and don't remember the exact context of the scene, but that was definitely what I heard.)
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If you have one character whose brilliance (or lockpicking skills or hacking or angst or whatever) is Their Big Thing at which they surpass all others -- then suddenly, all too often, the other people around them get made into incompetents, not even also-rans, who exist to be wowed by the one guy's skills. When often, in canon, Watson (or any number of others) was perfectly smart and perfectly competent, even if he wasn't in Holmes's league in Holmes's chosen specialization.
Of course, canons sometimes do this to their own characters too. (The above example of Dean from Supernatural is one I saw nearly all my friendslist up in arms about; they felt that all characterization said that he should know that perfectly well, and he got dumbed down to make other characters look better even though it made no sense. I don't watch Supernatural, so I don't have a strong opinion on it and don't remember the exact context of the scene, but that was definitely what I heard.)