I had a long think about this before replying because by and large my experiences with cabin crew have been extremely positive. I have felt that often the crew's attitude can make or mar a flight but also a good crew member can turn a negative into a positive.
I can remember my first VS flight which was in EC. It was a terrific experience which instantly bonded me to VS. OK, lots of things went wrong but it was their attitude in putting it right and the personal service which instantly wedded me to them. On the return I can remember my feeling of disappointment that the crew, while totally professional were not as upbeat or as helpful. All they seemed to want to do was feed us and put us to sleep! Now bear in mind this was to JNB so they were both overnight flights so the fact it was night time should not have come into play. In fact the next few return flights seemed to follow the same pattern but then all of a sudden it changed and I have had some of the most memorable inbound flights

What have been the positives? Being made to feel that you are the only person that matters - even in a full EC cabin. It's easily done with eye contact, a smile and being visible by not disappearing behind the curtain once the meal service is over. By clearing up quickly after a meal so that one is not left sitting in the debris of a meal. In UC being addressed by name makes all the difference. I have also noticed that VS are great with children and I just love it when they come on the PA and say "Ladies and gentlemen, boys and girls..."
The negatives? To be honest there have been very few negatives with VS cabin crew but the one that stands out the most is one who was very bossy towards passengers. When trying to prepare the cabin for take-off and landing there are ways to make sure that people are doing what is necessary. The same cabin crew was also very rude and dismissive of my teenage son in UC and I have had the misfortune to travel with this cabin crew twice and witnessed the same behaviour not only with him but with other passengers.
In summary: Your primary task is for our safety. That is a given. However, hopefully that aspect of your job will not come into play in the interval between take-off and landing. Therefore for that interval we want to feel valued and cared for regardless of our class of travel and during the few minutes of interaction with you made to feel that we are the most important person at that precise moment in time. That's a helluva a lot to ask for but easy to accomplish with the suggestions made above - a smile and eye contact.