You can always catch the common cold or flu by kissing and mono. As for "diseases" cold sores on the lips or in the mouth are a form of the herpes virus and are very contagious and can be spread through kissing or even sharing chap stick! Hepatitis B and some forms of bacterial meningitis could also be transmitted through kissing, but the cases are very rare!
To be on the safe side, don't kiss anyone who is sick or who has blisters on their lips!