That's the clincher isn't it. Any mob can work at any level, sure, that's true. But IMO, when you start having stuff like Dracoliches and deep ones being defeated by IMO, low-level adventurers, they lose their epic charm.
I don't see why. My protagonists - the PC's - do epic awesome things right from the start. It's easy, really easy, to make whatever else they do bigger, grander, and more epic than what they did before. I don't see how I gain anything by making the early levels less heroic than the latter ones.
My 'epic tier' is intended to be from 1st level on.
A 3rd level adventurer in my worlds is a fairly common sight. By 5th level they are only barely starting to thin out.
Stop right there. If in fact 3rd level adventurers are fairly common, why in the world don't they handle whatever problems arise instead of the poor 1st level mooks that are your PC's? I very much dislike the whole 'I'm too busy'/'I can't be bothered'/'I want you younguns to prove your worth' excuses for why the level 65 town fathers and gaurds don't go and clean out those troublesome level 1 kobolds next door. Likewise, I hate the general, "Over the hill, everyone is 50th level, but we can't find any one from there to take care of this goblin infestation!" that infests some settings.
In my campaign world, a 3rd level character is fairly ordinary. But such a character is usually a 3rd level commoner or expert with 15 pt buy and skills, equipment and feats geared toward overcoming problems of farming or commerce. Even if the 3rd level character is from a combative class, such as the veteren fighters in the king's standing army, he'll still be built with about 15 pt buy and he'll simply lack the knowledge, experience, and equipment you'd expect of a 3rd level PC fighter. He's quite capable as a soldier, but much less capable as an adventurer. The same is true of the out of shape hedge wizard who makes the town potions. Yes, in my world, you need to start being 5th or 6th level to really start standing out on the basis of level alone, but 5th or 6th level is about as high as you expect to find NPC's in my game and again most of this will be NPC classed with relatively low point buy equivalents and with a design that is not optimized for combat.
In my game, generic anythings are never 10th level. Tenth level characters are among the most powerful individuals in entire nations. They are never 'mooks'. You don't ever find rooms containing generic 10th level fighters simply because the party is 12th level and 1st level fighters would no longer be a challenge. The most infamous pirate in the whole world is probably a 15th level character.
Compared to most of the rest of the world, even from 1st level, the characters are extraordinary both individually and most especially as a team. The PC's are never mooks, and its not unusual at low levels for many of their foes to be higher level than they are and yet not necessarily as capable. While a starting group of heroes is not as powerful as some of their mentors, foils, and potential villains initially, they will be immediately recognized as having extraordinary potential. In my game, a 1st level PC is made from a 32 pt buy and is an advantaged character possessing a destiny. In my world '10' really is average, unlike some worlds where you'd be hard pressed to find an NPC with less than a 12 in anything. For there to be 4-6 such extraordinary characters working together is considered amazing by any NPC that observes it, and by 2nd or 3rd level they are a force and by 6th level they are generally well on their way to being legends.
Another thing to consider is that with NPCs level is usually tied directly to character age. A 4th level character is usually middle aged. A 6th level character is often a senior citizen. So to find such relative youths possessing such extraordinary skills is very rare.
I hate when fantasy monsters become mundane, and generally, I regard everything below 5th level as fairly mundane.
A self-fulfilling prophesy.
IMO, when "use rope" no longer becomes useful because a player can teleport, and the GM wants to make things a little tougher, I like to throw in a little something that inhibits teleporting.
While there is a place for that sort of thing, giving a player abilities only to take them away when they would be actually useful is antagonistic DMing and tends not to be very fun in the long run. It's much better IMO to up the ante so that the problems that they face require their greater power and ability to overcome, and so 'use rope' remains relevant to the extent that it lets you conserve your stronger abilities for when you truly need them.