Try this:
SELECT t1.username, t2.attribute
FROM userAttributes AS t1 INNER JOIN Attributes AS t2 ON t1.attrFK = t2.attrID
WHERE t1.username = @username
[EDIT #1 - missing attributes]
SELECT attribute AS MissingAttribute
FROM Attributes
WHERE attrID NOT IN (
SELECT t2.attrFK AS AttrID
FROM userAttributes AS t1 INNER JOIN Attributes AS t2 ON t1.attrFK = t2.attrID
WHERE t1.username = @username)
[EDIT]
More:
IN (T-SQL)[
^]