Revisiting this thread to provide playtest feedback
(I wasn't entirely clear whether this or
that thread was most appropriate for this post)
It concerns what I ended up with instead of Great Weapon Master, and specifically the Savage Attacker component of the replacement set of feats.
Savage Attacker
Once per round on your turn, when you roll damage for a melee weapon attack you made with two hands, you can double the dice of the weapon.
I can now say that this is probably too good in the hands of a variant human - that is at levels before martial characters gain their second attack. 2d6 is too reliable as a source of significant damage.
Also, it probably is too generous to take the simple route - which allows its dice to be doubled on a crit. Just like smite, the player can decide to apply its dice when he sees he has scored a critical, so this happens (slightly) more often that I thought.
Also, it probably is too generous to waive the "heavy" keyword requirement. At (much) higher levels there are a few specialist weapons that are versatile but definitely not meant to be combined with greatweapon fighting, which in this case means this feat.
Finally, it's boring how the Greatweapon Fighting fighting style all but steers you towards Greatswords, since you gain 2d6 which you reroll 1s and 2s for.
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But what I think is that all of these issues evaporate if we simply say that you only get one bonus die, and that die not being too large either. Then getting to double a crit can be accepted. Using it with versatile becomes okay. It can't be abused with the weapon style. And your choice of weapon is freed from the burden of optimization: meaning that, yes, a d12 is very slightly worse than 2d6... but as the playtest borne out, the real difference is often between 2d12 and 4d6... and if you get to reroll 1s and 2s that is no longer a negligible difference. Not to speak of crits or other sources of extra weapon dice!
All in all, here's the refined version:
Savage Attacker
Once per round on your turn, you get to add a d10 to the weapon's dice when you roll damage for a melee weapon attack you made with two hands.
As you can see, the actual difference is not huge.
The feat remains cleanly worded, with no attempt to restrict level 1 variant humans. Now that the feat is reined in somewhat, the extra damage is not as pronounced or reliable.
The doubling on a crit isn't prevented either. Again, the d10 might not seem much lower than 2d6, but really, with the reroll 1s and 2s there is a clear difference.
A d10 is also a "good" die size in that it minimized the impact of rerolling 1s and 2s. It simply makes it less necessary to use the feat with a greatsword. It makes it less of a bad choice to use it with another weapon.
The lowered die size (and lightened interaction with the weapon style) help justifying not bothering with "heavy".
Finally the d10 is the die size you get when wielding the Longsword as versatile
Zapp
PS. I did consider making it d8. And I'm listening if you want to argue the spell is still attractive enough if we make it a d8.