Monsters and traps, and the treasure that come with them are a big part of playing dungeons and dragons, after that you’d have to say the other main things of interest are the locations you visit, the powers you obtain from leveling up, the stories that are weaved through play and the fun and company of your friends around the gaming table.
Monsters are fun, or at least they should be, they can be sources of jokes, fear, and or good motivators to want to make something go splat,or to be the hero and right the wrongs. Let me expalin how I use different monsters to do different things in my games.
Goblins, are small little creatures and quite weak, good minions but they can be fun when they are given wolf mounts, flying giant bat mounts, or some other interesting items like some alchemical weapon. In large numbers they can be fun too, they are easy to kill so not too dangerous, but by having lots of them you can give relatively low level adventurers a ‘big fight’ as such.
Orcs make good soldiers for evil armies, they are evil and ugly and big enough to be a bit of a threat, they seem to like to organise into clans with violent sounding names and they are a favourite of parties to kill.
Ogres and giants are good for when your party are getting stronger, they are the big stupid monsters that pack a lot of fire power, the party can try to use their stupidity against the monsters and possibly have fun with this, it also has fun roleplaying elements for the dungeon master.
Minotaurs make for more intelligent versions of the big bad brute monster. Drow are a favorite of mine for intelligent oraganised foes as they have magic, levels and a large society. These creatures come across as quite cool and very evil, thus they make for good foes. Mind flayers and Beholders fall into this category too, however Beholders tend to be more loners, and like liches and dragons make for good monster bosses controlling some human run evil organisation.
Elementals make for good magic resistant guardians, they can hang around for centuries as well, golems also fall into this category. Undead make good guardian hordes for the same reason that they can hang around for a long time, without food or getting bored etc…
Undead are also great for atmosphere, they can be spooky or digusting, and they range from being mindless cannon fodder up to very intelligent foes, everyone loves a good vampire staking adventure!