So, yes, Barbarian with Reckless Attack is what makes GWM truly potent.
No, GWM is potent for any character planning to use 2-handed weapon. Often it's more powerful Ability Score Improvement for those characters, sometimes it's slightly less valuable. But if in release version of BG3 you will be able to take 3 ASI/feats, there will be nearly no reason to take any other feat for 2-handed weapon wielding character.
Also, if they DID implement Shove Prone, GWM would become quite potent especially at Level 5.
Shove Prone is either inconsequential or detrimental to power level of GWM. The former if you don't ever use Shove Prone and instead use all the ways to get advantage that are better*, the latter if you try using Shove to prone enemies instead of attacking them with your big sword or giving your martial advantage with better abilities*.
*
1) attacking enemy while hidden or otherwise unseen.
- We still can hide with a bonus action as any class (BA hide should be removed imho);
- Attacking target without darkvision from shadows, while having darkvision ourselves;
- Invisibility grants advantage on attack rolls (however we need Greater Invisibilty to make it reliable).
2) attacking prone enemy when within 5 feet.
Some ways of knocking enemies prone:
- frozen surfaces,
- Grease,
- Wildheart Barbarian Eagle and Stag special attacks,
- Beastmaster's Boar companion special attack (it's spammable, long line, with decent DC),
- Battlemaster's Trip Maneuver,
- Topple attack from Quarterstaff (14 DC, not attack roll, try it out),
- Tasha's Hideous Laughter,
- special weapon abilities that knock prone (Warhammers and Mauls have Backbreaker special ability and Intransigent Warhammer has Impulse Blast).
3) attacking blinded enemy.
- Fog Cloud, Darkness spells and Darkness arrows create clouds that blind enemies inside. You can place them in a way that a melee character could blinded enemies with little trouble. You can also build a Githyanki Warlock to actually stay inside darkness and attack with your Greatsword, could be fun;
- Blindness spell, Raven familiar and Beastmaster's Dire Raven, Absolute Night spear can blind enemies.
4) attacking restrained (emwebbed, ensnared, entangled) enemy
Sources of restrained:
- Web spell, Beastmaster's Spider. Give your melee character Spider Step boots and profit;
- Entangle spell from Druid, and Ensnaring Strike (Ranger, Sorrow glaive, Nature's Snare quarterstaff).
5) attacking stunned, paralyzed or petrified enemy.
- Hold Person is the only way to get one of those conditions, so not much here. We need to wait for higher level spells (and monks).
6) other sources.
- Off Balance condition from Rush Attack (Halberd, Longsword) or Flourish (Shortsword, Rapier) special weapon abilites.
- first attack against enemy hit by Guiding Bolt;
- Invoke Duplicity from Trickster Cleric (but who would have trickster cleric in their party?);
- Reckless Attack (just one attack per round in BG3);
- Attacks rolls against targets affected by Faerie Fire.
Shadowheart rushes up, shoves enemy prone. Lae'zel uses GWM while enemy is prone. -5 to hit but Advantage. +10 if she hits. Probably kills goblin with 1-hit KO. Then attacks a second with a second attack, and even a third attack with BA.
Picking Shadowheart as a example here is funny to me. Shouldn't you just use her Channel Divinity ability here?
So yes. In these specific circumstances GWM becomes very powerful.
Truth is that only in very specific circumstance - the enemy AC is higher than 14 (16 with bless, 19 with advantage at level 4) and there is little chance that you can get a killing blow at least once - choosing GWM is slightly worse than picking ASI.** There's never a realistically imaginable scenario where picking GWM would leave you a lot worse for any encounter since power attack is a toggle. In scenarios when GWM is better the effectiveness increase is insane.
I'm not saying it's not a potentially powerful feat. I'm just saying that it isn't necessarily OP.
My disagreement with your misleading arguments, not overall conclusions.
It actually reminds me of a lot of Larian magic items. IF you manage to do X and maybe even Y, you can then get the benefit, but you have to also make sure you activate it first.
Difference is that GWM is becoming even better when you are actively doing things that make you better regardless, while some Larian's magic items give you the benefit when you do something that makes you worse off. I personally like those items, but you can't compare being incentivized to get advantage on attacks to bringing your HP to less than a certain threshold.
