Personally, I have always been conservative with warnings (I'm a long-time Freeper. What would you expect?), and tend to set the threshold rather high. Normally, warning forecasters want to see more than just rotation in a thunderstorm. We prefer to see vertical and temporal consistency. That means the rotation occurs over a vertical depth, rather than one level...and over a period of time, rather than just once. The threshold will vary depending on the situation. If it is a wild day with a high threat, the threshold will be lower. You can't take a lot of time to analyze one storm when there are 10 others going on at the same time. You have to decide, and move on to another storm. If they are moving at 60 mph, you also don't have time to dither. Warning decisions are usually made in a matter of seconds.
I always preach that the toughest decision is the one NOT to warn. This is because if you are considering a warning, then you have evidence of a threat. You must conclude that the evidence is not strong enough to go with a no warn decision. Now you are betting the public’s life on things we don't fully understand. The easy approach is to just issue the warning and “be safe”. Of course, this just waters down the value of warnings.
Bottom line, it is a tough job. There is so much uncertainty, and often conflicting information. You are gambling with real lives. I have had numerous fatalities during my watch, but fortunately they all occurred with warnings in place. I have been lucky. I have watched other warning forecasters not so lucky. They will carry the scars of the wrong decision to the grave.
I remember when a local TV station installed their new Doppler radar back in the early 80s. One Sunday morning their tracking and warnings surely saved many lives - mine included - as they tracked a storm that hit (among other places) Mannford, OK. Though lives were lost (as I recall), I'm certain that more lives would have been lost were it not for the efforts of Gary Shore in Tulsa.