The main thing that irritates me is the way he recycles witty lines. (I'm sure most people wouldn't notice, but I love Oscar Wilde and so I notice it EVERYTIME.)

eg:

Importance of Being Ernest:
Jack: You don't think there is any chance of Gwendolen becoming like her mother in about a hundred and fifty years, do you, Algy?
Algernon: All women become like their mothers. That is their tragedy. No man does. That's his.

A Woman of no Importance:
Lord Illingworth: People's mothers always bore me to
death. All women become like their mothers. That is their
tragedy.
Mrs Allonby: No man does. That is his.

or:

Woman of no Importance:
Lady Caroline: These American girls carry off all the good matches. Why can't they stay in their own country? They
are always telling us it is the Paradise of women.
Lord Illingworth: It is, Lady Caroline. That is why, like Eve, they are so extremely anxious to get out of it.
Lady Caroline: Who are Miss Worsley's parents?
Lord Illingworth: American women are wonderfully clever in
concealing their parents.

Dorian Grey:
"Who are her people?" grumbled the old gentleman. "Has she got any?"
Lord Henry shook his head. "American girls are as clever at concealing their parents, as English women are at concealing their past," he said, rising to go.
...
"Is she pretty?"
"She behaves as if she was beautiful. Most American women do. It is the secret of their charm."
"Why can't these American women stay in their own country? They are always telling us that it is the paradise for women."
"It is. That is the reason why, like Eve, they are so excessively anxious to get out of it," said Lord Henry.

There are quite a few lines between "Woman" and Dorian Grey that are similar, eg American dry goods, when good Americans die they go to Paris. Mostly American jokes.

I dunno, it just bugs me when I read them!