As I live in the United States, where we have legalized a slave wage for servers because they MAY receive gratuities...
1. How much do you tip for average service?
--15-20%
2. What do you consider to constitute "average service"?
--Polite, timely service, with few delays within the server's control (I never hold the server accountable for the kitchen's issues or if there are delays caused by inconsiderate customers they might also be serving)
3. How much do you tip for less than acceptable service?
--15%. Once again, servers in the US are paid a slave wage, less than half the legal minimum wage. Furthermore, EVERYONE HAS BAD DAYS. Maybe my server is injured and is moving slowly. Maybe they received some terrible news earlier in the day and are emotionally distraught. Maybe it's just one of THOSE days where no matter how hard they try, nothing seems to go right. That totally means we should make their day worse by ensuring that they receive absolutely NOTHING for the time or effort they've invested (even if it's not to a good result), because that's totally how Christ would do it, yeah? Oh...wait.
4. Is it ever okay to tip nothing to your server, and if so, when? If not, why not?
--No. See above. I may not be Peter or John, and cure someone's lameness, but the least I can do is offer a blessing that might put food on their table or gas in their car.
5. Should you tip more if you were difficult (had finicky kids, a complicated request, etc)?
--Hm. I don't know. I actually make it a point to not be difficult. Those poor people put up with enough as it is.
5.1 Should you tip less if you were pleasant (acted very politely, altered your order to make the server's job easier, etc)?
--No; once again, less than minimum wage.
6. Any other thoughts on the subject?
--Sure. If our government did not allow for restaurants and bars to shirk the responsibility of paying their employees an ACTUAL WAGE, this wouldn't even be an issue. Seriously. By it's very name, a GRATUITY is supposed to be a gratuitous payment, something I should not feel obligated to give. But because I am aware that the restaurant industry is allowed to pay these people less than $3 an hour, what I give is no longer gratuitous, but instead, an additional fee in order to make sure my server is properly compensated.
As you can see, I feel very strongly about this; were it within my power, I would undo whatever stupid legislation went into place to put servers in such a crappy place. The vast majority of the civilized world has this all figured out already: my server should be paid by their employers, and if my server is gratuitously amazing in their duties, then I can choose to be gratuitously gracious in thanking them monetarily. If I choose to tip, it should automatically be a blessing, rather than an obligation.
Servers are not lesser people. They do not deserve to be treated as such.