The first D&D character to die in my old campaign was the not very originally named Hercules; he was killed by a fire beetle. This began my love affair with dungeon vermin -- giant rats, centipedes, spiders, and similar creatures. For some reason, they seem right, which is to say, they fit the kind of faux grittiness that I associate with D&D's default style of gameplay. Sure, everyone would much rather go toe to toe with goblins or orcs or whatever, but nothing says "old school" like getting the blood sucked out of you by a giant tick.
Dungeon vermin are, I think, a necessary extension of Gygaxian naturalism. They're "transitional monsters" that mark the dividing line between the mundane ecology outside the dungeon and the fantastical one inside it. I find monstrous vermin to be, frankly, far creepier than stuff like bugbears or minotaurs, because they could be real. They exist in that twilight world between the unlikely and the impossible and, let's face it, you never know what zoologists are going to pull out of the jungles of Indonesia or the rainforests of the Amazon next. There are already some disturbingly large spiders and insects out there and, while the square-cube law prevents them from attaining D&D sizes, that's cold comfort to a guy like me, who's more than a little squeamish when it comes to creepy-crawlies.
Some will no doubt argue that dungeon vermin are not "heroic" but, for me, that's a feature and not a bug (no pun intended). I remain firmly committed to the notion that 1st-level D&D characters are not heroes -- at least not yet. Poor old Hercules did not die a hero's death, but his death is remembered, if only because he'd waded into a combat, club in hand, that he should have avoided. His death was an object lesson in poor planning and the perils of leaping before you look. After all, how dangerous could a bunch of over-sized phosphorescent beetles be, right?