Absence of hassle in the departure airport is definitely way up there on everyone's list. As Evening Star rightly says, if there is any it just starts things off badly.
Another good thing is good information. There are always going to be problems sometimes. Even if there is bad news, I can take problems much better if there is truthful information as opposed to patent lies or, perhaps worse, just silence.
The rest depends to an extent on what sort of flight it is. I like and appreciate different things on a short hop from London to Paris as compared to the double-overnight haul from London to Sydney. It's difficult to pull common strands together because they are such different situations.
I think that the best crew are those who are cheerful and positive in themselves, but who are nevertheless sensitive to what the individual wants - which is often to be left alone. The open attitude will spread in the aircraft, but it shouldn't be so extrovert as to feel pestering or intrusive to those who don't want to engage.
Also, some people have the knack, when talking to you, of making you feel like you are the only and most important person in the entire world. That is a quality which always goes down well.
One thing I hate is crew (or, more often, ground staff) who fail to notice that I fly frequently and I know what I'm doing. I don't throw my FF status around for the sake of it, but there are times when I get into stupid arguments with staff members who tell me things which I absolutely know from experience to be wrong - eg about which check-in queues I'm entitled to use. I don't blame them for not knowing, because that's a training issue. It's the condescending way in which some staff members deal with these problems that's the irritation.