A decision point in dungeon design is anything where the players have to make a meaningful choice: Do I pull this lever? Do I try to balance over this narrow bridge? Do I open this chest? Sometimes these decision happen out of combat, but having decision point in combat much enhances the experience. A fight that happens in some special environment where the terrain gives modifies movement, or gives cover, or confers advantages or disadvantages, ends up being a lot more memorable than the same fight against the same monsters in a bare room.

Now 5th edition Dungeons & Dragons has a strong "old school" vibe, and a lot of the dungeons you can find consist mostly of bare rooms with monsters and the occasional treasure in it. Also the monsters in 5th edition are more frequently rather simple, having just an armor class, hit points, attack, and damage values. 4th edition, which was too complicated for a mass market and was played at lot less than 5th edition, made more frequent use of tactical influence of terrain and nearly every monster had some special powers and unique attacks. 5E combat is faster, but can easily feel a bit generic and repetitive.

So what I am trying to achieve is the best of both worlds. Using 5E rules and thus relatively fast combat, I am using 4E encounter design principles: Terrain interaction, environment manipulation, and monsters with special abilities. That includes using more homebrew monsters that are variations from some original in the Monster Manual. The result is combat encounters that are different, more epic, and where you can't use the same standard tactic every time. I just don't tell them what edition those design principles are coming from, because 4th edition has such a bad rep. See? I'm holding the 5th edition Players' Handbook, so this must be 5E! :)