I prefer variety, not the same sub/genre/style for an hour or more. 'Artist/producer' djs do this I find, 'DJ' djs not so much, like Randall etc, so I prefer 'DJ' djs personally with a very few exceptions, like Fierce etc.
Definitely this, and it becomes more and more important the less 'melodic' you are playing. You can get away with playing vocal/melody based dnb in the same style for a good while, because the variety comes from the musical content. If you do one hour of straight up neuro, murky techy rollers or just reese and amens it's going to get samey fast. And it gets worse the harder you play, because you can only hit people in the face for so long until the face goes numb.
Secondary things - knowing when to bring drops, when the crowd is ready for breakdowns, and just getting the whole 16/32/64/128 thing as slick as possible.
Unless you're playing the prime time party set, you can't base your set on providing a new huge double drop every 64 bars. The audience will wear out.
Similarly, if the set is interrupted by a 2 minute breakdown every 2 minutes, your audience will never find a groove. If you play for a dancefloor, you have to provide something people can move their feet and hips to.
Critical nights are a fuckin nightmare for this because they seem to tend to ramp it up to begin with, then right before the banner act drop in an hour of half time or some wonky broken beat business and it throws the whole vibe off so you spend the first 20 minutes of so of the headline set trying to find a centre again.
I guess that's why you never really see an opening DJ smash it, you're never like oh aye I better be back to see them because you need control of the vibe, a proper buildup through the night and then that earth shattering top of the slate set that ties it all together.
Programming a proper night is an art in itself, and it also depends on DJs playing their slots correctly. This is especially important for the earlier slots, because they prepare the audience for the rest of the night.
If you're the first (and possibly second) DJ of a regular club night, your job is to make people getting into the groove. That might very well mean you'll have noone dancing, because at the start of a night a lot of people will be nipping to their drinks, greeting friends as they enter the venue and ease in to the mood.
The measure of success is if people while doing this are nodding their heads, and perhaps moving towards the dance floor and standing in the back of the room to listen while doing it.
Also, remind yourself that when the first dancers start to appear, that is not your cue to throw away whatever you're doing to hit them with some 'proper bangers'. Just make sure you continue to play music that's danceable, maybe throw in some classics or nice bootlegs or whatever can bring a smile on people's faces and keep the groove going.
Finally, you should (if possible) try to have an idea of what the next DJ is going to play, so that you can use your last two or three tunes to set him or her up. If you provided the next DJ with more people on the floor than when you started, and the next DJs first tune ups the intensity and engagement from the audience a bit, you've done a great job!