[Bruno]

D&D 4e went a ways towards making a varied encounter quickly by identifying some general combat roles, and labeling monsters with them. I think it went a bit far by being very prescriptive, but the role isn't actually a game mechanic.

Basically it loosely groups monsters into Artillery, Brutes, Controllers, Skirmishers, and Soldiers. Artillery monsters excel at raining death from a distance, but avoid melee. Brutes are big meleeing sacks of hitpoints that hit hard but have low skill. Controllers change the state of the battlefield (make hazardous terrain, provide cover, move people around, heal, etc). Skirmishers are fast and lightly armored hit-and-run mobility fighters. Soldiers engage toe-to-toe like Brutes but seek to block movement rather than explosively destroy.

A crucial game-mechanical difference between brutes and soldiers is that brutes take damage smoothly and deal it out spikily, while soldiers take damage spikily and deal it out smoothly. IE brutes get hit almost all the time, but they have so many HP that individual hits don't matter as much - soldiers hit almost all the time, but they don't do tons of damage so individual hits don't matter so much. Meanwhile brutes rarely hit but deal a ton of damage when they do, while soldiers rarely get hit but suffer it rather more due to less HP.

Relative to a brute - a soldier still has more HP than any other role, usually.

Even vaguely classifying monsters into categories like this means you can look at an encounter and say "Oh, this fight is all skirmishers. Let's throw some soldiers in too to try and make the PCs stand still while getting harassed by skirmishers." Or, you can say "This fight is all skirmishers, lets put lots of interesting terrain around for players to chase them around/corner them in/get shoved into by monsters doing a hit-and-run".

That right there will give a lot of variety even if your skirmishers are orcs, and your soldiers are orcs, and your brutes are slightly bigger orcs.