[color:"orange"]To give you an example : Don't say "big demon" when you can say "demon which is composed of living organs of its victims, surrounded by all kinds of spiritually bound minions which it eats whenever its feeling weak and which is 10 times larger than the player".[/color]
This reminds me of the Ravenloft concept: You can create an eerie atmosphere by describing places and creatures in a special way, so even simple monsters like kobolds aren't just "kobolds", but "gaunt, diminutive beings that look like twisted children with a tortured look in their eyes". Of course, it's hard to do something like that if the player just sees the monster in a CPRG, but you could occasionally create suspense this way, e.g. if villagers are terrorized by some creature they never fully see and aren't able to describe properly. The player won't know exactly what to expect until the character ventures to the creature's den. This won't work for the standard monster you kill by the dozen, though.
I also think that monsters shouldn't just have characteristical looks, but characteristical behaviour, too. If you can talk to them, they might be haughty or humble, easy to trick or trying to manipulate the character, they might have a special way of speaking or another means of communication altogether. If you fight them, they shouldn't all just run towards you and hit or bite until they are killed. Most normal animals should try to flee if severely wounded. Other creatures should be shielded or immune against certain ways of attack, and that includes certain types of physical attacks, not just magic - ghostly and mist-like beings might be immune to all non-magical attacks, for example. Intelligent creatures should use special attacks against the player character, so you have to adapt your tactics accordingly.
