More often than not, written sarcasm is mistaken for a real adverse remark. Spoken sarcasm is recognized as such because your tone and your body language tell the other person this is a sarcasm. In a written sarcasm, there is no such clues. So, you should add some smiley (which is body language translated into ASCII art, after all).
I know about that, I have been bitten by this mistake already. Hope this improve your communication skills. No hard feelings, no "--".