I like class and subclass abilities that:
A) are reasonably straightforward without being overly fiddly or confusing (ie: user friendly); B) are powerful enough to be worth keeping track of but not game breakingly powerful (so: worthwhile in a balanced way); and C) are evocative of the character, class, or subclass's narrative. I also really like abilities that D) lend themself to creative play or have outside the obvious usage, but don't expect every ability to do so (at least not within a 5e+ framework). A good class or subclass feature fulfills at least the first three criteria reasonably well, though being weak on one point can be made up for on others.
So a pair of examples that come to mind for me from the Hunter Ranger subclass (because their names are, for little good reason, linked):
-A feature I mostly Like: Hunter's Prey. Extra damage to an injured enemy or an extra attack against an enemy next to another. Either option you can pick with it easily satisfies criterion A of being easy enough to understand, track, impliment, etc. and criterion B of being worthwhile but balanced, since both options are pretty powerful, especially at 3rd level, but they are also likely to only work in a few rounds per combat if you're lucky. The abilities themselves are great for criterion C, with Colossus Slayer going right to the whole hunter thing and Horde Breaker always making me think of Aragorn outnumbered by the Uruk-Hai, but the concept that you can switch options on a short rest makes no sense. This switching also makes it more complicated, and frankly wastes half the ability because unless you know you're to be up against a lot of melee weenies you probably never pick Horde Breaker (mileage may vary if your DM is always clumping enemies together, the point is that almost every combat has Colossus Slayer opportunities and many have no Horde Breaker ones). The switching is better perhaps on this front of seeing both get use than the old 5e version where you just picked one forever, but this improvement has come at the cost of making no damned ludo-narrative sense. My main design suggestion would be to simply allow either to be used on a per turn basis, or just make them separate abilities.
So, I dislike Hunter's Prey.
Colossus Slayer is an ability that is ok at level 3, but quickly falls off to irrelevancy. Tracking that 1d8 remains the same work at every level, yet becomes less and less important.
Horder Breaker is a "DM may I"; having foes be within 5' is almost always a tactical decision by the DM; do they want you to use such abilites or not? Tightly packed hordes aren't really well handled by the 5e engine either. If you removed the 5' restriction? Sure, great ability. Even if you made it 15', it would be quite usable. As is, it is an ability that doesn't see play.
Neither really make you play your character differently; maybe a melee PC with Horde Breaker might move to a different spot 1 fight in 20. So it has no impact on the character's combat loop.
The level 7 abilities aren't much better. Escape the Horde is an ability that makes a situation you should avoid being in a tiny bit less worse, and Multiattack Defence makes combat for the DM annoying (but only when they attack you with a boss type; which, I guess, is a kind of defence?)
Superior Hunter's Prey is, I agree, poorly designed. Magical what damage goes where?
The entire subclass is just "I fight good" and then doesn't really deliver. Even Hunter's Lore is a combat ability; it has the only real flavour of the subclass.
Hunter's Prey:
Collossus Slayer: At the start of your turn, you can increase the damage of your Hunter's Marks on a creature by 1d6 on one creature who is not at maximum HP you can see. You can only increase the damage to 2d6.
When you reach 7th, 11th and 15th level in this class, the max number of 1d6s your Hunter's Mark can be increased to goes up by 1d6 (up to 3d6, 4d6 and 5d6).
Break the Horde: When you use Hunter's Mark, you can place it on up to two creatures. Once per turn, when you attack a different creature within 15' of a creature you have Marked, you can make a weapon attack on the creature you have Marked.
When you reach 7th, 11th and 15th level in this class, the number of creatures you can place your Hunter's Mark on increases to 3, 4 and 5. In addition, you can make the extra attack twice per turn starting at level 11.
Level 7: Defensive Tactics
Escape the Horde: When a creature subject to your Hunter's Mark makes an opportunity attack on you, you can expend your reaction to make a weapon attack on it first (if this is a ranged weapon attack, it doesn't have disadvantage for beign adjacent to foes). If your attack hits, the triggering creature's opportunity attack misses, and all other opportunity attacks made on you until the end of your turn are at disadvantage.
Multiattack Defence: When a creature subject to your Hunter's Mark damages you the 2nd time on a turn, you can expend a reaction to make a weapon attack on the creature; if a ranged weapon attack, it is not subject to disadvatange from being within 5' of a foe. If you hit the creature with this reaction, the damage from your attack reduces the damage you take.
Level 11: The Hunt Eternal
Hunter's Mark no longer requires concentration. When its duration ends, the spell effect remains, except you can no longer apply it to a new creature when the creature dies.
Level 15: The Hunt Inevidible
By spending a bonus action you can learn what direction a creature subject to your Hunter's Mark is in, and optionally move up to your speed directly in that direction. Attacks you make on that creature gain advantage until the end of your next turn. Creatures subject to your Hunter's Mark do not impose disadvantage on your attacks from being unseen, nor gain advantage on attacks on you from being unseen.
I think I mixed in a bunch of flavour and non-combat utility with the hunter's mark improvements. This subclass plays
differently than other ranger subclasses; it has a different combat game loop etc.
Break the Horde ends up with 5 creatures under HM per cast and 2 extra attacks per turn. Collossus Slayer ends up with a 5d6 (!) damage hunter's mark, but takes 4 turns to get there.
The level 7 features are offensive-defensive instead of pure defensive. They only apply to HM targets; one advantage of the Horde option is that more creatures are going to be subject to your HM.
The Hunt Eternal adds a pile of non-combat utility to HM. You can put it on a foe long before you fight them, or have it up on a foe that fled.
Level 15 changes the entire story dynamic, and is a fun and useful in-combat ability.