One thing I like to see is adventures that at least try to address different ways PCs might try to deal with an encounter and try to account for different outcomes. Obviously, there's only so much can be done on either front, but at least some effort is appreciated.
I would second the "NPCs with good motivations" suggestion above.
I think these two things go hand in hand. If you know what the hostile NPCs want, it becomes much easier to work out alternative resolutions. For example, in the goblin ambush at the beginning of Lost Mines of Phandelver, the goblins are motivated by greed. They want to ambush travelers, take their stuff, and bring it back to their hideout. That encounter got a lot of notoriety for being very difficult for the first encounter of an introductory adventure, but it’s really only dangerous if you run it as a fight to the death. If you keep in mind that the goblins aren’t actively seeking to kill anyone, but are willing to if someone tries to stop them from taking the valuables from the cart, it becomes a much less deadly encounter.
Something that I do in my own prep that I never really see in adventure modules, but would be incredibly DM friendly in my opinion, is to lay out the goals, sources of conflict, and dramatic questions of encounters. So rather than just saying “there are four goblins hidden in the thickets on either side of the trail, they attack the PCs as soon as they reach the ambush sight,” it would be much more DM friendly to lay out
PC Motivation: The PCs are being paid to escort a catload of mining supplies to Phandalin.
Source of Conflict: Four Goblins are lying in wait to ambush travelers on the road.
Monster Motivation: Take anything of value from the cart and bring it back to Cragmaw Hideout
Dramatic Question: Can the PCs prevent the goblins from escaping with the mining supplies?
That changes the whole dynamic of the fight. It informs the goblins tactics, since their motivation is to escape safely with the PCs stuff, they’re less likely to waste time attacking the PCs, and more likely to Dash, Dodge, and Disengage to try to nab as much as they can and get away, only attacking PCs who block their route to the back of the cart. They might throw torchs in the cart as they run away in hopes the PCs will choose to deal with the fire rather than give chase. Most importantly, by explicitly stating the Source ofbthe conflict, it makes it easier for the DM to work out other ways that conflict might be resolved besides everyone on one side or the other being killed or otherwise incapacitated. And by stating the dramatic question, it makes it easier for the DM to recognize when the question has been answered and the encounter should be wrapped up.
