I remember when I first started playing D&D. Like many earlier players of the day, my initiation took place in the Caves of Chaos, Keep on the Borderlands. I remember being so excited and scared about everything I encountered in those caves. I didn't know what a goblin or orc or a kobold was and everything around the next corner was a deadly threat. I remember getting the snot kicked out my party by a group of 7 orcs. Orcs at the time seemed like the scariest thing imaginable. We ran......a lot. In those days, when dungeons were new and monsters were still scary to encounter, even encountering an orc was a thrill.
But now it seems, the thrill of encountering most monsters these days is gone as seasoned players aren't as fearful of the monsters I set before them. Often I hear " oh, its a troll, get out the torches", " don't touch the rust monster with your metal weapons", and even worse...... "oh, its just more orcs".
Humanoids really weren't all that different back then either- an orc or kobold or goblin or gnoll were pretty much all the same- just a humanoid to kill- some types possessing more hit points than others. It was good to see later editions of the game try and give the humanoids each their own encounter personality such as Tuckers kobolds and Pathfinders goblins.
One thing Ive tried to do to spice things up for players, is to change up the abilities monsters have. It spices things up when they come across a gelatinous cube that's touch causes shock damage and can teleport about, or divide in two. Or the Troll that breathed fire and was only damaged by cold, or the kobolds who could spider climb. Ive seen even the most jaded character turn a deep shade of "what the fuc...", and suddenly that glimmer of fear the unknown is seen in their eyes.
Presented here is a table of monsters I mostly pulled from Basic/Expert book and a list of abilities those monsters might possess. Randomly roll for the monster then for its Special Ability or choose to suit your needs. I don't suggest you do this all the time, but maybe once in a while to keep the players on their toes. Nothing is more fun than when your players come upon a room of trolls, and roll their eyes as they order torches to be lit only to find out not only are those trolls immune to fire, but their breathing fire down on their asses....