It seems to me that one of Rowling's strengths as a writer is in writing characters for us to be annoyed by. The Dursleys are constant throughout, obviously, Dobby, as mentioned above, and now the cheese-fest and hack Gilderoy Lockhart. There's at least one in every installment.

It is entertaining to read the different reactions to him, from the young male perspective (Harry and Ron's incredulity) vs. Mrs. Weasley, Hermoine, and the sighing that ensues.

The confrontation between Lucious Malfoy and Arthur Weasley in Florish and Blotts is amusing as well. Stepping in to diffuse the situation and actually getting into a "muggle duel," made for a good laugh.