I don't think that StringMatchQ["x", Except[{"*"}]] works as expected as well as StringMatchQ["x", Except["*"]]:
StringMatchQ["x", Except[{"*"}]]
StringMatchQ["x", Except["*"]]
True
True
The string pattern "*" is an abbreviated string pattern consisted from the only metacharacter * which corresponds to zero or more characters according to the first point under the "Details and Options" section of StringMatchQ. So by the definition the pattern Except["*"] shouldn't give a match at all because it means negation of literally everything. Also for matching verbatim "*" one must escape this metacharacter with double backslashes "\\*" (the reference is the same). So current behavior is a bug.
The following is a set of examples of the expected behavior:
StringMatchQ["", "*"]
True
StringMatchQ["x", {"*"}]
StringMatchQ["xy", "*"]
True
True
StringMatchQ["*", Except[{"*"}]]
StringMatchQ["*", Except["*"]]
StringMatchQ["xy", Except["*"]]
False
False
False
StringMatchQ["x", "@"]
True
StringMatchQ["x", {"\\*"}]
StringMatchQ["x", "\\*"]
StringMatchQ["*", "\\*"]
False False True
The following is a set of examples of wrong behavior of Except with metacharacters:
StringMatchQ["x", Except[{"\\*"}]] (* metacharacter is correctly escaped *)
StringMatchQ["x", Except["\\*"]] (* metacharacter is correctly escaped *)
StringExpression::invld: Element Except[{\}] is not a valid string or pattern element in Except[{\}]. >>
StringMatchQ["x", Except[{"\\*"}]]StringExpression::invld: Element Except[\] is not a valid string or pattern element in Except[\]. >>
StringMatchQ["x", Except["\\*"]]
StringMatchQ["x", Except["\*"]] (* metacharacter is wrongly escaped *)
StringMatchQ["*", Except["\*"]] (* metacharacter is wrongly escaped *)
True
True
StringMatchQ["x", Except["@"]]
StringExpression::invld: Element Except[@] is not a valid string or pattern element in Except[@]. >>
StringMatchQ["x", Except["@"]]