I am working on building monsters for skyRPG. I am trying to avoid a lot of the obvious screwups that have been made over the years (Beholders! They die in one round and have a broad range of instant death attacks! Huzzah!) with various monsters in a variety of games, though obviously sometimes avoiding one bad decision leads you to make another instead. I am having a bit of a philosophical debate though, surrounding the DnD 2nd edition or 4th edition monster styles.
In the good ole' days, monsters were presented as a fact of life. They had an XP value to give you some idea of how dangerous they were but the XP values were not well correlated to their difficulty and it would be very easy to wipe out the party (or make the fight a cakewalk) by putting in the wrong types of monsters even if the XP total looked right. This feels like a much more real world to me, where the monster manual is not a system designed to generate appropriate encounters but rather a description of something concrete.
The 4th edition style is to give monsters levels, types, and special power sets based on whether or not you are supposed to encounter them alone or in packs. This certainly makes it easy to make challenging but fair encounters (in theory, the system at launch was horribly flawed) but things really don't feel immersive somehow. When a random mercenary I meet at level 20 is automatically 5 times as powerful as a mercenary I meet at level 1 things feel really bizarre. It isn't a world we are exploring anymore, but rather just a jumble of numbers.
So how should I design monsters? My current plan is to have players and monsters each have a Encounter Strength value that is, in theory, the same scale for both. Players have a ES equal to 30 + Level so 4 monsters of ES 80 would be a 50/50 proposition against a Level 50 party. The idea would be that GMs would generally put encounters at roughly 66% of the character's ES against them - the characters are rated to win nearly always if they play reasonably. Harder encounters could go as high as 90% of the character's ES, and if the GM really wants to end the campaign they can always just send an enemy with noticeably higher ES than the group.
This never works out quite right though; it isn't idiot proof. Skeletons swing for 2d6 damage, so once the party has 6 Armour or so they are pretty near invincible. In theory 27 Skeletons have an ES high enough to be a dangerous fight for a max level party (Level 50) but in practice the Skeletons are a complete joke. They take a while to blow up because there are a lot of them but when an enemy does 1 damage / round to you they really can't be a threat.
I guess I am kind of leaning towards the DnD 2nd edition style. Monsters are going to be presented as an entry in a tome of information with a single stat that describes how tough they are overall. This will lead to some fights that don't work well, but as long as I stick warnings in there about populating high level fights with enormous numbers of complete dorks it should be okay.