So, yes, Barbarian with Reckless Attack is what makes GWM truly potent. Also, if they DID implement Shove Prone, GWM would become quite potent especially at Level 5. 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.
Or... Faerie Fire a group of enemies. Lae'zel with GWM gets Advantage on a few of them.
So yes. In these specific circumstances GWM becomes very powerful. I'm not saying it's not a potentially powerful feat. I'm just saying that it isn't necessarily OP. 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.
