I have a little different perspective as I and my wife were servers for one of those major chain bistros for a couple of years. Yes, we had to wear 'flare' so if you don't know which one you need to watch Office Space. As such we do tip 20% and work our way down for bad service, but it depends on WHY the service is bad.
You can tell 'clueless' from 'apathetic' from 'arsehole' pretty quick.
Clueless doesn't bother me much because odds are they are still learning or just stupid in general. The second most common bad service we have run into has been of the clueless variety. Its really pretty funny because while we may not have been servers for going on 15 years now, we still know why they are floundering and what their mistakes are as they make them. We had one very memorable poor server at our local favorite establishment because of the clueless server. The guy was maybe 24, about 6'3, and blond so he was easy to follow around as he worked. Our table had a view of the kitchen through the big glass partitions on a packed Saturday night. He of course completely screwed up our table and apparently every other table he had. We would watch has the managers would talk to him, the cooks, other servers and he had that look of beyond in the weeds the entire time. We actually gave him a 15% tip because we felt sorry for him as he was doing his best, even if his best was awful. We never saw him there again.
Apathetic is the norm for 'bad service'. You can tell them pretty quick on how they act when they take your drink order. These are the type where if male you will see them spend more time hitting on the female wait staff than paying attention to their tables. Usually the service is just adequate, not poor but just enough to not have major complaints, odds are we tip this kind a bit too high.
Arseholes are pretty rare and those are the only ones where we do the .01 tip or complain. I can't recall the last time, and I doubt these people last in the business long enough to really run into a lot of them.
As a waiter I screwed up a few times, one I even feel really bad about still today (forgot a birthday), but things do happen. Over all being a waiter is not a difficult job but it does require good multitasking skills to really pull it off. Most quality people will not be life long professional waiters, so yes this is a part time job while we are in school for other things. Some of the posts in this thread seem to imply that it makes the sever somehow less important because of this, but really who wants to be a sever their entire life? I've always said everyone should work at least 6 months on tips alone, it will greatly improve peoples attitude to others.