I don't see how these descriptors are useful additions to the concept of level. If you want to challenge a party of four level 5 characters, you throw four level 5 opponents at them, or one opponent of several levels higher. One arbitrary metagame mechanical description of overall power is already more than enough.
If every character and monster just had a "power" stat (or several such stats that all worked the same but never crossed), then you'd be onto something. But in D&D, they don't. Hit points are somewhat independent of AC. Damage is somewhat independent of attack bonus.
Thus given the bounded accuracy attempt in Next, having some kind of mechanical thing like "elite" and "solo" makes even more sense than it does in 4E. Namely, they seem to be sticking to the idea that attack bonuses and defenses stay within rather narrow
but still increasing bands, while increasing hit points and damage are the main representation of overall power. (In 4E, with no real limits on any of the factors, a few named stages didn't really cover all the ground.)
So "level" of a creature is where it more or less follows that overall formula. Elite and solo is when a creatures misses out on whatever minor but real increasing attack and defenses would go with a higher level, but nonetheless gets a big boost in hit points and/or damage. (Damage is the one they have been most inconsistent with yet. I'm not sure they've got it fully hashed out.) Presumably, elite and solo also are the ones to get the abilities that make them more of a threat against multiple PCs at once and/or change the nature of the encounter by their very presence.
The closest anyone has ever come to using multiple dimensions of power and rolling them up into a single scale that actually works is when they do something similar to Hero System, where the "Active Cost" of a power is a guided balancing factor, being a product of both relevant dimensions. That mostly works in a rough and ready way, but it's unwieldy and very self-referential (i.e. only has meaning as a short-hand once you've grokked the nature of the dimensions). I suppose one could do that for "level"--with it being the multiplication of factors derived from defense, attacks, hit points, and damage, but I suspect most people would find it far more foreign and counter-intuitive than the proposed method.
