I'll jump in...naturally being in the business, I give the server much benefit of the doubt, and consider whether it seems like he or she has been trying his or her best. However, a bad or dismissive attitude will surely affect the amount I tip.
What gets me is when they drop the main course and don't follow through with the condiments and refilling the drinks right away... Now I'm waiting for them to come back while my food gets cold... You shouldn't have to wait 5 or more minutes for the ketchup or steak sauce to come so you can eat your food... Then make sure that I have enough to drink so I have something to wash it down with while I eat...