One of the most basic ways that people mess up kissing is by going to extremes with the amount of pressure they use. Some people are way too forceful with their kisses, and wind up bumping teeth, pushing their partner backwards, or coming off as aggressive.
Other people are shy and timid, and their kisses end up feeling lifeless. Think about handshakes: it feels strange when someone squeezes your hand way too tightly, and kinda creepy when you get a limp handshake. You have to use some pressure but not too much.