Monsters in old versions of DnD were incredibly dangerous. They would charm characters, turn them to stone, poison them, disease them, pop out of the ground, or any number of other unpredictable things. In DnD 4th edition that is not at all the case and pretty much every monster presents a predictable numerical challenge. A medusa is terrifying at any level in old school DnD but in 4th edition it is a joke once the characters outlevel it a bit because there is no more danger of instant petrification. The old model more closely mirrors the real world since after all there are plenty of poisonous critters that will kill you dead, dead, dead if you disrespect them and avoiding them has nothing to do with combat skill and everything to do with knowledge and preparation.
The crux of the matter is whether or not players actually enjoy the realism of being randomly killed because they didn't know a monster's particular powers or weaknesses. The world does feel more dangerous when you will die instantly if you don't get out a mirror / get fire ready to stop regeneration / stuff your ears with wax but it does feel a little ridiculous that out of character knowledge is so paramount. Hydras aren't exactly common so how is anyone supposed to know that you can't beat them without burning their heads off first? Does it make any sense that characters have to know that harpy songs will charm them without hearing protection and failing to do so means everyone dies? (Or, you know, harpy sex slaves or something.)
When the game is played as a simulator where characters are just vehicles for players to 'win' then all of these crazy critters seem justified. If you don't want to die to the first weird monster that shows up then read the entire monster manual and memorize that shit! I don't much like those sorts of games though as I prefer to be in character and to have my character act as a person in that situation would. Well, as an irrationally brave, combat oriented person in that situation would at any rate; they really shouldn't just sit in the library learning cool stuff for the whole campaign and I know people who would do that! Really I like the idea of a character who doesn't know all these things and has to muddle through anyway so having monsters that require out of character knowledge to defeat bothers me.
I have been building a bunch of monsters for HBT and I want to avoid monsters being too generic so this problem has come to the front of my mind. I decided to build a hydra that has a ton of HP and regenerates those HP very rapidly to reflect its heads regrowing. It doesn't require special knowledge to defeat but the mechanics definitely support a terrifying creature that heals any damage done to it moments after that damage is dealt. Once you chop off the last head (deal actual wound damage) to the hydra it dies so the combat should, if described properly, reflect the legend of the hydra. The characters don't have to have special tactics but they do have to beat the hydra down hard because if they are too defensive the beast will simply outheal any damage they do to it.
The next step is for me to create a bunch of weird conditions monsters can inflict on players. At the moment monsters have a huge variety of tactics and abilities but they don't inflict hideous and lasting suffering. While I don't want instant death attacks and petrification to be usual I do think that a disease that slowly transforms someone to stone so that they need to rush to find someone with a Ritual that can cure it is a cool idea. The ideal situation to me is one where the players have to find solutions to problems that monsters pose without randomly dying to them right off. If monsters do nasty stuff to characters but are then defeated the situation becomes a new challenge that can be overcome and that strikes me as a lot more fun.
One thing I really like the idea of is making these afflictions linked to taking wound damage. If players only take HP damage they avoid being poisoned or diseased but if they take real damage something terrible can happen. This seems like it has to potential to alter strategy and really increase the stakes even if the players don't actually suffer from the affliction in question. Combining these ideas seems like it could lead to some really great strategic choices where players have to figure out how important it is to defend their friends vs. going all out and bashing monster faces. Now I just have to do the actual work of implementing all of this.