1. Suds in your suds
There are few things more beautiful than the foaming head of a well-carbonated beer, and few things grosser than bubbles streaked down the sides of your pint. Those bubbles mean your glass is not clean. Instead of rising to the top, the carbonation gets stuck on the sides because of some type of debris -- dirt, grease, or sanitizer (if you're lucky). Congratulations. You will never enjoy draft beer again.
2. Tap touching
The faucet of a tap should never touch the glass. Would you want to put your mouth on that beer tap? Well, maybe, but it's not sanitary... the exterior of the faucet collects all kinds of germs, including all the ones it procured when you put your mouth on it.
3. Foaming over the faucet
Another cardinal sin of cleanliness. Some bartenders think that, in addition to touching the tap to the glass, you should let the beer rise over the faucet in order to reduce excess foam. This soaks the faucet in foam, thus making it even more of a germ powder-keg... a powder keg that just left some powder in your beer.