The Monster
Explorer
A rule of thumb I've been using for years now is that "players want to do cool stuff." Whether that means raw combat power, special tricks, or other things, it pretty much always includes opportunities to use all those special powers and abilities they have built into their characters. Encounters that are easy can be fun, but rarely call upon any special stunts; it's the ones that push firmly but reasonably against the players, so they have to do a little more work than button-mashing their basic attack/skill, those are the ones that work best in my experience.All encounters should be:
1) Interesting
2) Meaningful
I don't want to waste any table time on "uninteresting" or "meaningless" encounters.
Oftentimes, "difficult" falls under the classification of either (1) or (2) or both of them. Difficult could be "measure of challenge" or "costly in some non-fungible way (eg you aren't simply trading resources for success)".
Rarely, "difficult" falls under neither of those classifications. If so, find a type of "difficult" that is either/both "interesting/meaningful."
Any GM can make a hard encounter; any GM can make an impossible encounter. A good GM can make an interesting encounter.