jgsugden
Legend
You do need to remember to put these capabilities into context.
Dueling, for example, can add 16 damage to your attacks during a round at 20th level (with an action surge - maybe 20 with haste and a reaction attack)... but to benefit from it you must be wielding a weapon that inherently deals less damage than our high water mark two handed weapons. It takes you from so-so damage to pretty good damage.
Great Weapon Fighting, which does less than you you suggested (your average six sider goes from an average of 3.5 to 4.1666666, which means a great sword goes from an average of 7 to 8.3333 which is only a 1.333 increase while a great axe goes from an average of 6.5 to 7.3333 which is only 0.8333333 on an increase per attack), pushes the envelope. You're talking about one of the highest damage options out there and you're increasing that damage to even higher levels. It wasn't an oversight that it adds less damage - they were very intentional about it. It would have been obvious and easier to make great weapon master better by just letting you reroll any damage dice. On a 2d6 weapon that would give you 4.25 damage per die if used on sub average rolls (for 8.5 average damage), and a greataxe would go from 6.5 to 8.58 - making them comparable to great swords ... but they intentionally did not do that and restricted it futher because they realized that great weapon fighters are amongst the most agressive at damage dealing in 5E and didn't want to push the envelope that far.
Blind fighting, is situationally useless - but if you build around it is insanely powerful. How would you feel if this feature was "all of your melee attacks are at advantage, your enemy's attacks are all at disadvantage, and you can't be the target of spells that require that you be seen". This is pretty close to what we saw with an Eldritch Knight Drow build that focused on obtaining a Ring of Obscurement at low level (fog cloud 1 to 3 times per day). 3 to 5 Fog Clouds per day - on a build that was decently stealthy - at level 3. The most common combat start was to spy, drop fog cloud or darkness as a surprise and then charge in and start killing. The entire party built around it and it became their iconic method of getting in there and killing foes. At higher level they were all using darkness and seeing through the darkness.
Protection is underrated. Let's say you've got an ogre attacking an ally within 5 feet of you and the ally has an AC of 17. At a +6 to hit, that ogre will critical 5% of the time, and hit normally 45% of the time. A critical will deal 18, and a normal hit 11. Average damage is 5.85. Now, if you give that attack disadvantage, the chance to get a critical hit is 0.25% (1 in 400) and the chance to get a normal hit is 24.75%. (99 in 400). The average damage drops to 2.7675 - A savings of over 3 damage - every round, potentially. While doing so, the chances of that high damage critical that just flat out kills a low level PC are nearly eliminated. Further, this scales at higher levels as enemies deal more damage. If an AC 25 ally is being attacked by a storm giant you are potentially saving them from 16.05 down to 7.68. That is like a first level healing spell per round. Healing is often less effective than damage dealing (cure wounds heals 1d8+ wisdom while an attack usually gets to add magic weapon damage, bonus damage from spells like divine favor, and can potentially start at 2d6 rather than 1d8). I'm not saying this isn't one of the weakest options ... but it is not so far behind other options like many people think.
My suggestion for DMs that think some of these need augmentation: Don't change them. Instead, give your PCs an opportunity to earn a boon from a task performed for a church and allow that 'Boon' to augment an underpowered ability for each PC. You don't mess with the rules, and you instead give them PC something that makes them feel special.
Dueling, for example, can add 16 damage to your attacks during a round at 20th level (with an action surge - maybe 20 with haste and a reaction attack)... but to benefit from it you must be wielding a weapon that inherently deals less damage than our high water mark two handed weapons. It takes you from so-so damage to pretty good damage.
Great Weapon Fighting, which does less than you you suggested (your average six sider goes from an average of 3.5 to 4.1666666, which means a great sword goes from an average of 7 to 8.3333 which is only a 1.333 increase while a great axe goes from an average of 6.5 to 7.3333 which is only 0.8333333 on an increase per attack), pushes the envelope. You're talking about one of the highest damage options out there and you're increasing that damage to even higher levels. It wasn't an oversight that it adds less damage - they were very intentional about it. It would have been obvious and easier to make great weapon master better by just letting you reroll any damage dice. On a 2d6 weapon that would give you 4.25 damage per die if used on sub average rolls (for 8.5 average damage), and a greataxe would go from 6.5 to 8.58 - making them comparable to great swords ... but they intentionally did not do that and restricted it futher because they realized that great weapon fighters are amongst the most agressive at damage dealing in 5E and didn't want to push the envelope that far.
Blind fighting, is situationally useless - but if you build around it is insanely powerful. How would you feel if this feature was "all of your melee attacks are at advantage, your enemy's attacks are all at disadvantage, and you can't be the target of spells that require that you be seen". This is pretty close to what we saw with an Eldritch Knight Drow build that focused on obtaining a Ring of Obscurement at low level (fog cloud 1 to 3 times per day). 3 to 5 Fog Clouds per day - on a build that was decently stealthy - at level 3. The most common combat start was to spy, drop fog cloud or darkness as a surprise and then charge in and start killing. The entire party built around it and it became their iconic method of getting in there and killing foes. At higher level they were all using darkness and seeing through the darkness.
Protection is underrated. Let's say you've got an ogre attacking an ally within 5 feet of you and the ally has an AC of 17. At a +6 to hit, that ogre will critical 5% of the time, and hit normally 45% of the time. A critical will deal 18, and a normal hit 11. Average damage is 5.85. Now, if you give that attack disadvantage, the chance to get a critical hit is 0.25% (1 in 400) and the chance to get a normal hit is 24.75%. (99 in 400). The average damage drops to 2.7675 - A savings of over 3 damage - every round, potentially. While doing so, the chances of that high damage critical that just flat out kills a low level PC are nearly eliminated. Further, this scales at higher levels as enemies deal more damage. If an AC 25 ally is being attacked by a storm giant you are potentially saving them from 16.05 down to 7.68. That is like a first level healing spell per round. Healing is often less effective than damage dealing (cure wounds heals 1d8+ wisdom while an attack usually gets to add magic weapon damage, bonus damage from spells like divine favor, and can potentially start at 2d6 rather than 1d8). I'm not saying this isn't one of the weakest options ... but it is not so far behind other options like many people think.
My suggestion for DMs that think some of these need augmentation: Don't change them. Instead, give your PCs an opportunity to earn a boon from a task performed for a church and allow that 'Boon' to augment an underpowered ability for each PC. You don't mess with the rules, and you instead give them PC something that makes them feel special.