Comparitive threats

Okay, so hands up who goes numb at the thought of setting up balanced monster encounters in D&D?
The D&D approach of phone books of creature statistics always bugged me.  Coming from tunnels & trolls, where monsters just get a Monster Rating (MR) that defines their hit dice, I always found the D&D way of having an appropriate list of monsters to throw against players at certain levels a little bit daft.  I want two displacer beasts showing up but... hmmm... can the players take these on?
Apart for my penchant for Displacer beasts, I love beholders and Aspects of Tiamat.  I never get to use them in D&D because my players are usually only on a few months stay and never get up the level.  Sure, whacking kobolds is fun for a bit, but most of these guys are experienced and want some variety.  I could kick them off at higher levels, but then they miss out on their early adventures.
Why, as a lowly adventurer, can't I take on a beholder... just one thats a little bit shit?
Why take the time to get the exact right number of monsters to match up against a party of adventurers, and what happens when you have a wide mix of pc levels?

So it occurs to me that one neater way of tying up the threat encounter is to treat monsters as difficulty ratings or, like T&T, just give them a level.  A threat level.  The exact attacks and powers of the creature are layed out, and their appropriate threat levels- but ultimately you can set your level at whatever you like.
A level 4 player can take on a threat level 4 and do well.  A level 8 threat would wipe the floor with them, leaving a small gap to overcome and a larger one to escape... just.
The beauty of a threat level system is this... any scenario can work for any level of player character.  You simply have to change a number.