Defensive Duelist - Functions against all attacks in an attack action.
Hm. Problematic if the character is facing only one opponent, but then that's the point of the feat, isn't it? I can see this being too good in campaigns that favor single monster encounters. As long as it's only melee attacks I'd be willing to test it, but my gut says this is a bad idea.
Durable - A 1 on a Death Save does not count as two failures.
Sure. An insignificant benefit to an underpowered feat, IMO. I'd rather it be something like "The first time you fail a death saving throw (even a natural 1) you instead treat it as a success. You must complete a long rest before you may use this feature again." That makes you a
lot harder to kill, though, since it's essentially 4 failures before 2 successes.
Great Weapon Master - Raises your melee weapon damage dice one step. Does not offer -5/+10
I'm not a fan of universal damage bonuses any more than I am of making players do cost/benefit analyses in melee. I'd rather something like, "When you're fighting a creature that is at least one size category larger than you and at least Large, your attacks with Heavy melee weapons deal an additional 1d4/1d6 damage." A good feat should enhance the character and flavor of the subject, not just make it flat out more powerful all around.
Alternately if you want a universal damage bonus, increase the minimum damage by making 1s count as 2s, or (in the case of d10 and d12) 1s and 2s count as 3s.
Keen Mind - Once per long rest you can end a charm, enchantment or fear effect on you as a reaction. You have advantage on madness saves.
I don't know what "once per long rest" means. Like it's actually ambiguous. 5e usually words things as either, "During a long rest, you may do XXXX as an action," or "As an action, you may do XXXX. You must complete a long rest before you may use this feature again." I'm not sure which you mean here.
Beyond that, this seems kind of like a Wisdom power to me.
Lucky - Cancels disadvantage rather than reroll.
Hm. I'm philosophically opposed to anything that eliminates advantage or disadvantage. Get disadvantage from 10 sources, advantage from 1, and burn a luck point? Bleah.
I'd rather reduce the feat to 2 luck points.
Martial Adept - Save DC is based on an attribute of your choice at the time the feat is acquired. You gain gain 2 superiority dice.
I disagree with the change here. Martial = Str & Dex. I don't see a problem with 2 dice, though. Superiority dice are already fairly marginal, IMO.
Moderately Armored - You do not get disadvantage on stealth checks in medium armor.[/COLOR]
Ever? For any reason? Your phrasing is a bit ambiguous, though I do know what you mean. I suppose this isn't bad, but personally I like players having to chose between +1 AC and stealthiness. I suppose adding "or take a feat" to that list isn't such a bad thing, though.
Sharpshooter - Raises your ranged weapon damage die one step. Does not offer -5/+10
Again, not a fan of universal damage bonuses. I'd rather, "When you're fighting a creature that resists weapon damage or piercing damage, your ranged weapons deal an additional 1d4/1d6 damage." There. Now you've got sniping for missing armor scales. Black arrow! I have saved you to the last.
Weapon Master - You are proficient in all weapons.
Yeah, that's what this feat should already do.
As far as my suggestions, frankly, I'm tempted to bar Polearm Master and Great Weapon Master from working together, and barring Sharpshooter and Crossbow Expert from working together. They're no longer interesting when they're that much better than everything else. I don't know of a good way to do that, however.