Thanks Joerg, that saves me a long post!
Generally, creatures with big Damage Bonuses, lots of armour or multiple attacks are more dangerous than normal oppnents. Skill isn't that important, unless it is vastly higher than 100%, an opponent at 60% is not much worse than one at 80%, in my opinion. Having magic makes an opponent much stronger, depending on the magic. Having an NPC:PC ratio of more than 2:1 makes the NPC Party dangerous, but anything less than that isn't really much of a problem.
By the way, sorry about the multiple posts. I don't generally check the forum until Saturday and then read through the threads, posting replies.