In general, I think a "boss monster" should exist because it's a lot tougher than the PCs, not because it has a keyword that piles on hit points and damage.
This seems to be confusing ingame and metagame. Ingame, the monster is tougher than the PCs. The mechanical technique whereby this is achieved is by piling on hit points and damage.
A level four goblin should be a "boss monster" to level 1 PCs because he's so much tougher than they are, not because he's a Goblin Manslayer [Elite].
Huh? What is levelling up a goblin, other than piling on hit points and damage, presumably to represent the toughness of that goblin compared to the ordinary goblin and the level 1 PCs?
But in a game based on an action economy in combat, like D&D, there are also issues about giving the "boss monster" a suitable suite of actions. So one way to "pile on damage" is to give multiple attacks at lower damage (a bit like the mechanical strategy that AD&D used for fighters, paladins and rangers). In an abstract combat system like D&D there need be no ingame difference between one attack roll for big damage and two attack rolls for moderate damage. We can choose between mechanical interpretations in the interests of gameplay convenience and pleasure.
if 5e is designed the right way, the boss tag should be mostly obviated.
Bounded accuracy should mean that low level characters fighting a higher level challenge will still be able to affect it. As such, a higher level creature will be (from a simplistic outlook) just a bigger pool of hp with more damage output (a simplified solo).
If done correctly, the hp threshold mechanic should prevent such bosses from being stun-locked to death.
There is still the issue of the action economy. Everything else being equal, I think multiple (or perhaps AoE) attacks make for more interesting play than big-damage single-target attacks, when there is only one enemy on the field.
Boss monsters are great. They are proven idea that has worked time and time again. The mechanical distinction between normal grunt enemies that are fought in swarms and single foes capable of handling a group of heroes all on their own is absolutely essential to pretty much any game that features enemies to fight. If D&D lacked that it would be fundamentally flawed and impotent.
Agreed, although I think fantasy RPG design has come relatively late to this point. There is an interesting discussion of the issue in the Burning Wheel Adventure Burner, for example (that book is a bit like a DMG for BW): they discuss various strategies (mechanical, encounter design, etc) a GM can use to help "bosses" survive the one-vs-many action economy. (And I don't think it's a coincidence that the same BW book that discusses this issue lists 4e D&D in its bibliography, as one of the influences on it!)
A monster stat block is one of two things:
*A set of modifiers that you can deconstruct and use to make a monstrous NPC.
*A handy example that you can modify to suit your needs if you don't have the time or inclination to deal with it.
For most of D&D's history, a monster stat block has been neither of those things. It has been a representation of certain capacities of the monster in a suitable mechanical form for usign that monster in game play.
in 1970 I think D&D was more of a glorified miniatures game and less of a storytelling system.
I don't want to play D&D as a "storytelling" system with massive GM force. I want to play it in a reasonably lighthearted but Forge-y style with an emphasis on thematically and mechanically strong scene framing supporting player protagonism. And I want monster building rules that support that.
I think WotC are going to have to think about more than just your preferences, or my preferences, in desigining a "unification" edition of D&D.