Lets take a look then what exactly the games do for combat.
Diablo 1. if you make a warrior then all you really do is slowly walk around and use basic attack. This game is completely carried by it's item system, atmosphere and story. Moving around and attacking, maybe dodging sometimes is really basic, most games have that. This really isn't a game that has good combat.
Diablo 2. If you make a barbarian then you start off using your basic attack, then get attacks like bash, frenzy (which is cool because it adds attack and movement speed) and whirlwind (which is really cool because it's one of the most iconic skills in gaming, even i add something like that to my games whenever i can). Until you get frenzy or whirlwind though, you are stuck with the same system that you have in Diablo 1. You just walk around, dodge and attack. This game seems to be mostly carried by it's RPG elements as well. At least this game has decent enemy variety with unique stats for enemies as well which change fights sometimes quite a lot. The problem with this game is you really end up using only a few skills, for example a barbarian only has shouts and an attack skill. Barbarian doesn't really need to deal with immunities either while other classes have to. A class like sorceress would do a lot better if you choose 2 elements, for example one to deal with fire immune enemies and one to deal with cold immune enemies. Sorceress is probably one of the most interesting classes in the game, they can get attack skills of 2 elements (3 if you cound thunder storm), energy shield (damage taken goes to mana), teleport (instant movement), chilling armor (defense and freezing enemies), warmth (regenerate mana), static field (reduce health of bosses fast), thunder storm (periodically make lightning fall on enemies). This game has okay combat but i feel it's possible to do a lot better.
This also brings up a few things the game does badly. Skill like teleport shouldn't exist. If all other classes have to walk, so should sorceress. This really makes sorceress a lot faster than other classes. Immunities shouldn't exist. There are way better solotions to make combat interesting. Sorceress gets almost all the tools you want which are good defense (damage taken goes to mana, chilling armor and energy shield), good movement (teleport), able to have skills of different elements against enemies weak to certain elements, mana regeneration (warmth), static field and good AoE (frozen orb has basically full screen AoE). Barbarian has very few upsides, they have good defense because of passives but it's kinda made irrelevant because of their bad AoE. Maybe Barbarians are good in party play but i'm the kind of player who plays single player only, so it doesn't really matter to me. The other thing i don't like about most RPGs is that they seem to be balanced around party play. Skills should probably work differently, if you're in a single player game than a party game.
Diablo 3. This game actually tries to offer you bunch of skills to use and has decent enemy variety. This game might actually be the best example for combat but the problem with this is that the game is spammy. You really just use your skills whenever they're available. Spam cooldown skills whenever they're available and buff whenever you can. When you're not doing that you use your main attack skill.
Path of Exile. This game makes you focus around one attack skill but offers utility skills, like curses, golems and warcries. I actually like this system most because it is least spammy. The game lets you get cool skills early on and let you build a somewhat interesting skillset. This game could do more with it's skills though. This game could easily have some of the cooldown skills they have in Diablo 3 but not too many, so it wouldn't become spammy.
For combat you do have to consider things like movement speed, attack speed, levels, controls, pathfinding, animations, sounds, graphics, enemy attacks and atmosphere as well. All enemies should really have a huge variety of attacks and animations. Dark Souls and Bloodborne do it quite well. Every boss kinda feels like a game on it's own. Those games have a great combat system. It required you to time your moves very well. You needed to attack and dodge at the right time. Enemies have weaknesses and you have to find them and use them properly. Maybe RPGs need to move towards that style more? Should enemies have attacks that you have to roll to avoid? Should Diablo like games have Dark Souls style bosses? One good thing about Diablo is that it is relaxing to play. Would the game still be fun, if it was a lot more difficult? Maybe there should be different type of maps, a map type that you can farm without much concentration and a map type that requires a lot of concentration to farm? Maybe more difficult maps offer better rewards? Diablo like games could introduce blocking as well. Maybe even combos that can be seen in games like Street Fighter? How complex of a combat system does a game like Diablo really need? The options are endless, i think it's important to find a good balance between fun and difficulty. Maybe the further you reach in the game the more difficult the game becomes? Maybe it starts out as simple as Diablo 1 then becomes as difficult as Diablo 2 then becomes as difficult as Diablo 3 then becomes as difficult as PoE then becomes as difficult as Dark Souls and then becomes as difficult as Street Fighter basically. For a game that has about the same difficulty all the time i think i'd prefer Diablo 3's skill system combined with PoE's character building system.
Sometimes adding things can ruin a game as well, for example Borderlands has great combat and RPG elements but it's ruined by some of the things it has. To even play the game you have to deal with repetitive quests, unskippable cutscenes and childish story. The game even has quests that force you to do nothing special for a few minutes. This adds up to hours of wasted time, if you play for a decent amount of time. This is a good example of a game loop being done very wrong. Borderlands would be a much more enjoyable game, if it for example simply used the map system that can be seen in PoE. Diablo 1 is a good example of a game loop done right, it doesn't have much meaningless content. Diablo 2 has some meaningless content, for example annoying areas, like act 3 and maggot lair and some quests. Diablo 3 doesn't have much meaningless content but it's a simple game. PoE has some meaningless content. I don't think the acts are even required in PoE, so you could just play maps which offer a lot more variety in it's current state but it's probably necessary for most players to get a decent experience out of the game. Dark Souls doesn't really have any meaningless content but it doesn't have any replayability. That's a game that doesn't really have any random generation at all. That's a game that could definitely use something like a map system.
History has shown that RPGs work well enough without the best combat out there but combat definitely helps. I think the most important thing after all is how all the systems come together. Which is your favorite combat system and why?