Everything. The mockery, the non-sequiters, the general tone of combativeness combined with breezy nonchalance. It's a pretty thorny style for a pen-pal. The letters read like he's desperate to communicate with someone.
I realize-- as someone already astutely pointed out-- that we don't know the other half of the conversation, which is important and might change how we read Moz's side, but these letters seem to me to be the work of a young man with "a murderous desire for love". "You're an illiterate with bad taste and I don't know why I'm bothering-- oh, do send me a photo!"
Don't get me wrong. I like the letters and find them funny. I'm completely aware Morrissey is simply writing in an amusing, sarcastic style. Just strikes me as someone who doesn't know how to make friends and probably suffers from crippling shyness and insecurity. Which is how Morrissey described himself back then.