Other things.

Terrain is damn hard to figure out, I think. Some situations favor either side. Some of these are easy, usually being at a higher altitude is a good thing, seeing your opponents before they see you is a good thing, and special terrain (beneficial or harmful) that one side will reach before the other, you can figure out which way that swings.

Traps are interesting terrain. Sometimes the enemy set them and knows to avoid them, in which case clever players can deduce trap locations and avoid them, or try to knock enemies into them. Sometimes monsters may not know about them though. And where they're placed, and what they do, plays a big part in how much they change the fight. A simple arrow trap is less of a big deal than a pit that requires a full turn to climb out of, etc.

Monster abilities vs. PC abilities, I mentioned earlier. Even though two monsters may be the same CR, their various abilities can lead to widely different difficulties. If a PC group focuses on single target attacks and neglects AoE effects, swarms are gonna be harder. If nobody is good at ranged attacks, flying creatures are gonna be terrifying. Monsters that work in packs (like wolves or hobgoblins) are scarier when you have a whole team of them. That kind of thing.

Oh, and while we're on the subject of monster abilities, use them. Low intelligence doesn't mean you stop using each monster's calling card. Even dumb hobgoblins know to stick next to each other, they're just not as good at figuring out where they should be standing, or when they should be moving, as smart hobgoblins are.

Oh yeah, if I want to customize my game a bit, I'll change a few ability scores from the stat blocks. Just up or down for a few scores, by a few points. So that, like I mentioned, there are such things as smart and dumb hobgoblins, strong and weak ones. Just enough so it's not all uniform.