Do I personally agree? Oh yes, definitely.
Would the average RPG player, even the average Codex RPG player? Not really. Even if we exclude the mainstream RPG player, the actual encounter part of encounter design isn't what people are looking for.
2) If so, what do you think are the keys to good encounter design, and what are the things to avoid?
I) In the prime position for what encounters need is relative power balance between the two opposing sides. This should absolutely NOT be parity in class/level/stats/number appearing. Expeditions: Conquistador is not the way to go. If all you do is fight a parallel of yourself over and over, the only thing left to cause a change-up from one fight from another is terrain. With proper parity, the enemy can have any number of differing variables from the party, but the combined power output of their variables must be close to the party's combined variables. And it must be so simply because that's the only way for the combat to ever actually be a challenge. Anything else is just pretend challenge. Of course, not every fight should be at full parity, as there should be ebb and flow to build tension and interest, but all encounters do need to be within a relatively close range of parity. For rest-anywhere modern games, the parity lines MUST be relatively close for all encounters. Attrition-based combat is better at this, as the ebb can be steep, with tiny easy encounters possible, there for the sole purpose of draining little bits and pieces of the party's resources (before the game goes in for the kill with the big battle).
On the other side of things, terrain is the primary component for player interest from the regular RPG crowd. In other words, a combat that not only occurs in a unique location, but in which the player must interact with that unique location in order to defeat the enemy. For instance, the tried-and-true arrow-slitted murder hallway. Throw in an a giant ogre in the center of the hallway whose attacks shake the screen (but who can't really do enough damage to defeat the party with his awesome swings), and you have the classic awesome encounter that everyone loves that feels challenging. It feels like you have overcame a tough situation, when the encounter was actually specifically designed as a kind of jump-scare. Ogre attacks shake the screen, arrows firing from places unseen and that you can't easily get to - spooky stuff. And then you slaughter everyone with your awesome special overpowered weapons and abilities. No real challenge, but it feels unique and special and challenging.
II) Secondarily, the enemy needs a reason and means to fight the party. They need some hook that can allow them to defeat the party, some attack or some trap, some something that makes it possible for them to actually kill the party despite all of the party's suped up weapons and powers. Or, in attrition-based games, they need some ability to damage the party at least slightly. If the enemy don't have this hook, then the combat is automatically just rote routine, essentially the party getting rid of a fly level of combat.
On the other side of things, fly-level is what regular RPG players want from their bulk combat, it's only their boss battles that they want non-fly-swatting duties. For the current crop of RPG players, divide encounters into three groups - minions, henchmen, villains. Henchmen and villains are the boss battles. Minions are drek. What people want is to walk all over the minion fights, stomp stomp stomp, dead. Then when the player encounters a henchman, that's when they have to think, not a lot, but just take a few extra moments of consideration to make a few extra clicks. Fail to do that, and the henchman has a chance of defeating the party. And then finally, there's the villains. The villain fights are where it's at, where players are required to make use of their full suite of abilities. Essentially what this all means is: your current core RPG player wants ebb and flow in combat quality, lots of stuff that they can scoot right over that makes them feel powerful, a few simple henchmen combats that make them feel both clever and powerful, and then only a handful of challenging villain encounters, with the villains spread out through the game.
III) Thirdly, the different combat encounters should require different sets of tools to complete successfully. Some encounters should start at long range, some at close. Some should occur in tight, close quarters where large weapons and ranged weapons can't be used, and where area-of-effect is king, others in wide open areas where the enemies are spread out and area-of-effect is nigh useless. The player should need to adapt to the situation, not be able to one-size-fits all power their way through every encounter with their awesome avenger, ember, vorpal two-handed sword of beast slaying.
On the other side of things, your current RPG player hates everything in that paragraph above. They want to be specced out with their favorite weapon and favorite armor and have all their favorite feats and powers and special abilities set to max, and use combat as just a kind of illustration of how awesome their character-building prowess is. Take any of their favorite things away, even just for a couple of combats, and they will declare your game to be the worst ever. What they want is to have a variety of types of encounters, each one of which can be bent to defeat with their favorite things. This requires a careful balancing act, since the combat has to be physically different and appear to be challenging, yet at the same time must be defeatable through any number of different methods, as long as the player has chosen to max out one particular method. That means your villains have to be powerful, yet highly vulnerable at the same time. Like an allergic vampire with poor eyesight and mental issues (deep mental issues, such as whatever it is that keeps him from adequately using his minions to protect himself).
