Let me add one line to this "definition":
A "well built" monster displays evidence the designer is aware of what tricks level-appropriate heroes might have up their sleeves.
Since low level heroes have basically no tricks, this starts out easy enough

But too many high-CR MM entries can be trivially shut down.
To not be a joke on a high-level battlefield you must have ways to either reach your opponents or to make meaningful attacks from afar. Merely a land Speed of 30 doesn't cut it - it's far too simple for any thinking group of players to kite such a monster and trivialize its defeat. It's that simple.
Do note that I'm not talking about rank and file monsters now.
I'm not even talking about the Death Knight. It's perfectly alright that SOME boss monsters can be neutralized by keeping them away from you (and he does have his hell ball after all).
It's perfectly alright if an individual grunt out of a dozen can be taken out of combat easily. Indeed, doing so is often critical to your party's continued survival
But for a "boss monster", such design simply does not cut it.
Guess it's high time for actual examples. One category of monsters that are particularly badly affected by this in this edition is demons, a group of monsters I have gotten to know quite well since I'm running Out of the Abyss.
The Marilith is described as a "general of demons", yet its stat blocks suggest a simple brute with no means of overcoming tactical hurdles. Many demon lords are likewise far too naively designed.
I get that the designers wanted to simplify the 3E stat blocks that often contained lots of minor stuff that simply never got used.
But they failed. They threw out the baby with the bathwater. Pruning away a dozen minor abilities is one thing,. but taking away crucial battlefield maneuvrability options
is a clear error.
As I said, if you compare the CR 8 Hezrou demon to either of the Diviner or Warlock NPC (from Volo) the difference in actual threat level to the party is so enormous it isn't even funny. And that's even before we even think about reshuffling those spellcaster's spell lists as has been suggested here.
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One more thing:
I believe spellcasting monsters should have had their most common spells statted out as monster abilities. This would help us DMs immensely, instead of assuming we know every PHB spell by heart.
I also believe such NPC spellcaster stat blocks must indicate concentration, right in their spell lists. It is a huge inconvenience to have to check each spell for concentration, which you must do if you don't want to outright cheat.
For instance, here's the spell block from the Mage NPC:
Cantrips (at will):
fire bolt, light, mage hand, prestidigitation
1st level (4 slots):
detect magic, mage armor, magic
missile, shield
2nd level (3 slots):
misty step, suggestion
3rd level (3 slots):
counterspell,fireball,fly
4th level (3 slots):
greater invisibility, ice storm
5th level (1 slot):
cone of cold
Imagine how much easier it would be to run this guy if his three most likely Cast Spell Actions were statted up as Actions, plus any reactions, and if any concentration spell were clearly marked. A quick glance at the following version tells me Fly and Greater Invisibility can't be combined, since they're underlined.
Like this:
Cantrips (at will):
fire bolt, light, mage hand, prestidigitation
1st level (4 slots):
detect magic, mage armor, magic
missile, shield
2nd level (3 slots):
misty step, suggestion
3rd level (3 slots):
counterspell, fireball, fly
4th level (3 slots):
greater invisibility, ice storm
5th level (1 slot):
cone of cold
Actions:
Cast Cone of Cold: 60 foot cone dealing 8d8 cold damage (Con save for half)
Cast Ice Storm: 20-foot-radius, 40-foot-high cylinder within 300 feet deals 2d8
bludgeoning damage and 4d6 cold damage (Dex save for half)
Cast Greater Invisibility: You or touched creature becomes invisible while concentration up to 1 minute
Reactions:
Shield: The mage gains +5 AC until the start of his next turn, including against the triggering attack, and becomes immune to Magic Missile.
Counterspell: The mage automatically counterspells a spell of 3rd level or lower, or makes a spellcasting check to counterspell (d20+6 vs 10+spell level) if the spell is of higher level.
As you can see stat blocks should make no attempt to reproduce spells in every detail.