I'm not really going to comment on how their suggestions would affect 5E though. Instead I'll consider here how they might work in Classic, AD&D, or other OSR type games.
1. Better Criticals: Use 4E style criticals, where you roll normal damage plus add the maximum potential damage from your weapon to the roll as a bonus, rather than rolling twice (or doubling the number rolled).
I don't use critical hits in my game. Classic works just fine without them. Mostly because hit point totals just aren't that high. And a few bonuses to damage from high Strength scores and a magic bonus give a weapon really good damage in this edition. An ogre (4HD) in Classic has an average of 18 hit points. A Fighter with 16 Strength and a +1 sword would deal 1d8+3+8 on a crit with this rule, dealing 12 to 20 damage. A normal hit is 1d8+3, or 4 to 11 damage. Yes, the crit will make a big impact, but not as big as in 5E where an ogre has around 80 hit points on average while damage per hit (at low level) is fairly similar. This makes sense for crits in 5E, not so much for crits in Classic.
If I were to use critical hits in Classic, I'd probably just make the attack deal automatic maximum damage, with no bonuses or doubling. If you score a crit, don't bother rolling for damage, just do your maximum. Because like they say, it's pretty anti-climactic when you roll a natural 20 to hit, then roll a 1 (if doubled damage) or snake eyes (if rolling multiple dice). Dean has been using this type of critical house rule in his 5E Eberron game, and it's working well there.
On the other hand, though (and this will be mentioned in #2 as well), the swinginess of the random die results is, IMO, a feature of the game, not a bug. So something that removes randomness from that game like this may also take something away. In the normal house ruled Classic/AD&D critical rule, where you double the damage rolled on a natural 20, the natural 20 is a "potential critical" in effect, and the damage roll determines if it actually is a critical or not.
3E actually had a sort of double jeopardy of criticals, where you had to roll a natural 20, then roll another hit roll to confirm the critical, then roll double dice for damage. THAT can lead to some disappointment, as you need to get lucky once, then twice, and three times in a row to get that massive critical damage.
My Opinion: Classic works just fine without critical hits, but if you do want them, simply maximizing the normal damage works better than rolling extra dice or doubling the amount rolled.
2. Better Healing Potions: This one is simple. Don't roll for healing potions, they just automatically heal the maximum amount of hit points.
Their rationale is that players feel cheated if they roll low for the amount healed, and it might not be enough to protect them from dropping to 0 on the next hit.
In Classic, though, as noted above, hit point totals and damage amounts are generally lower. So a Classic potion of healing's 1d6+1 points (2 to 7) is enough in most cases. Yes, rolling a 1 or 2 sucks, but again, there's tension riding on that die roll. You drink your potion and pray for a 5 or 6 to come up on the dice. That's exciting! Having an automatic 7 points healed is boring.
Now, in the PbP AD&D game I'm in, the DM has two house rules about healing. For potions, instead of 1d8 points healed (the rule in AD&D), it's 1d4+4. So you're guaranteed at least 5 hit points back, but there's still some swinginess to the result. For cure wounds spells, the minimum amount healed is equal to the Cleric or Druid's level, up to the maximum allowed by the spell. So a 3rd level Cleric casting cure light wounds will always heal at least 3 points, even if they roll a 1 or 2. At 8th level, your cure light wounds spells automatically heal the maximum 8 points.
I've started using this house rule for spells in my game, but potions I'm keeping by the book.
My Opinion: With the low hit point totals of Classic compared to 5E, there's really not much need for this house rule in Classic, but it will sure make the players happy if it's implemented.
3. Flanking: Already an optional rule in 5E, they use it but with a tweak. Instead of flanking granting advantage on a hit roll, they give a +2 bonus to the attack, similar to how cover grants a +2 or +5 to AC.
Now this is a house rule that I would consider using for my Classic game. I'm not sure it's quite necessary, but it might be useful. I'll have to think about it.
Of course, in AD&D, there are facing rules, so the character the opponent is facing gets no bonus but the character attacking from behind or the side gets some bonuses to hit. If I were running AD&D, I'd just use what's in the book. In Classic, though, and since I use theater of the mind instead of minis/battle mats, a simple rule like this would be easier to remember and implement. Flanking an opponent grants a 10% increased likelihood of scoring a hit? Sounds about right. And of course, monsters can flank the characters, too...
My Opinion: This one is worth considering for Classic D&D. It's simple, and can add some excitement to the battle by trying to maneuver into flanking positions or maneuver to prevent enemies from getting into flanking positions. And yes, a flanking backstabbing Thief should get a total +6 to their backstab attempt.
4. Bloodied Condition: This is of course a 4E thing. And as they say, it's most useful as a means of signalling progress to the players. Once a creature has reached 1/2 or lower hit points, they are "bloodied."
Now, in and of itself, there's nothing special about this condition. There aren't any penalties attached to it. But 4E had lots of character and monster abilities keyed to the bloodied condition.
I wouldn't want to add in a lot of special abilities for monsters based on this. It's just more book-keeping for Classic, where there aren't lots of special "exploit" type abilities (bonus actions, lair actions, legendary actions, recharging actions) like in 5E.
And just as a description of how badly the monster is beat up? I do that anyway. So without any actual mechanics connected to it like 4E had, it's not really worth adding to a Classic game.
My Opinion: As a descriptive quality, I already signal to the players how badly the monsters are damaged. It doesn't need a specific codified rule to be useful in this sense.
5. The Minion Rule: Another 4E rule that they're importing. Minions are monsters with only 1 hit point and deal average damage on a hit. They accompany a "boss" monster in packs and are designed to be distractions from that boss. Or they're designed to be thrown at high level characters in large numbers so they can wade through the army of orcs cutting them down like they're harvesting grain.
Now, this can make the game much more cinematic. Think of the scenes in the Lord of the Rings movies where Aragorn, Gimli and Legolas are slicing their way through Saruman's and Sauron's armies. Or a jidai-geki like Abarenbo Shogun where at the end of almost every episode, the shogun faces down a group of 30 to 40 samurai retainers of the villain of the week who rush at him one to three at a time and he dispatches easily.
I think, though, that in OSR games -- again, because of the lower hit point totals in general -- you already get this. Most of the humanoid opponents that make good minions, like bandits, orcs, kobolds, etc. already have low hit points. Sometimes they'll survive a hit if they got lucky for hit points and the player got unlucky for damage, but for the most part they're already mostly dropping from one hit anyway.
And again, making them do average damage may save a little bit of die rolling, but it again takes out the swinginess that I like.
And at high levels, do we really want minion ogres or minotaurs as goons for a lich or beholder, for example? Why not just keep using orcs? A high level Classic character will likely have good magical items that allow them to pretty much wipe out a goblin or orc in one hit anyway. While there's nothing wrong with it, the system default is fairly close to that idea anyway.
My Opinion: It won't hurt to use this in a Classic or AD&D game, but I'd still want to have the monsters roll for damage.
Overall Opinion: Taking some of these house rules into your Classic D&D or AD&D game probably won't hurt anything, but they will remove a lot of the randomness that I feel makes the game interesting. The Flanking rule is the only one that I'd consider adding to my game at this point. The others, in relation to the Classic game system, are either redundant or move the game away from its natural power curve.