Besides the usual "don't bite or scrape it with teeth", one thing I don't care for is when the sucker tries to deep throat but ends up not doing it right and putting pressure on the top of the glans and pushing the whole cock toward the body. While it's not painful for me it is unpleasant and uncomfortable.

On the "do" side of things, I do like it when the deepthroater knows what they're doing and it goes in there nice and smooth with no resistance. Also like getting a hand stroking my cock in unison with the mouth action, makes it more pussy-like which is just what the cock was made for. After some hot mostly-mouth action for a while this can be a good way to finish me off pretty easily if it's done decently well.