What were mundane abilities that could keep up woth 9th level spells?
I'm sure I don't know. It's not my job to figure out what those should be. It's the job of the people asking us to pay $50 a book (or is it $60 now?) for their rules, while telling us that Champion and Banneret are supposed to be equal to Paladins and Wizards and Bards.
I've dealt with the skill issue by changing my philosophy of skill checks. If something is painfully obvious that you will succeed, no DC on it. If something should be so hard a novice can't do it, the DC is over 20.
Ability (skill) checks are the most uninteresting thing in D&D.
Mostly because of how people refuse to let them do anything interesting (likely because many put too much emphasis on using spells to do interesting things). Though the excessively high DCs and abandoning the concept of Skill Challenges are also major contributors.
I don't think 4e did it all that well. There were abilities that crossed the line for many people.
Whereas I found 4e delightful in this regard. The Fighter is emphatically not magical; no Fighter power can do anything actually supernatural. The closest things were absolutely people hunting for something to get offended by;
Come And Get It is incredibly tame (as in... it's literally an assault on someone's pride, and missing the attack roll literally means your insults failed to land. There nothing remotely supernatural about it, people were just desperate for anything they could find to validate their false claim that 4e had turned Fighters into Wizards.)
The specifics on Spell design are in the DMG.
Spell casters have the same number of Skills, but nobody can have all of the Skills. Different party members get their moment to shine based on their Skill choices. Whi h, yes, is DM dependent, but that's the nature of D&D.
Uh...no, they don't. Bards get extra skills (and can get even more extra skills.) Clerics have several ways to get extra skills. Warlocks too. Wizards and Druids IIRC are the only full casters which don't have baked-in extra skill sources (will have to check some of the more recent subclasses), and Druid gets Wild Shape which already gives enormous skill-obviating powers.
The numbers are in the Dungeon Masters Guide. My personal experience and the published math suggest that you are overvaluing these magic effects as elements of game balance.
Howso? Where is the flaw in my logic? The effect of
hold person is to paralyze an enemy. Within two or three levels of getting
hold person, most characters are getting their first cantrip damage boost or Extra Attack, so characters are typically doing at least 12 damage per round with basic attacks (accounting for misses and crits, at the typical 60% hit rate) WITHOUT advantage. Add in the damage the monster cannot deal because it is paralyzed for at least the one round (if the spell lands at all, which it usually will), and I cannot see how anyone could rate hold person as only 3d10 equivalent (that is, ~16.5 damage), the equivalent of a moderately high-damage Fighter's average damage (again, counting hit rate and crits) for a
single round of attacks.
If you factor in that melee attacks will have advantage against the target, then a single decent-damage (1d8+3) melee character can deal 19 points of damage across two attacks, again accounting for hit rate (boosted by advantage to 84%) and crit rate (9.75%, which I subtracted from the hit rate to ensure no double-counting.) Meaning having literally just one character with Extra Attack 1 (so a Fighter, Barbarian, Paladin, Ranger, Monk, dual-wielding Rogue, certain types of Bard or Warlock, possibly some others I've missed) you can outstrip the alleged damage equivalent of
hold person. Hell, that's literally the same as claiming that hold person is equivalent to a CANTRIP at level 11 (when
fire bolt becomes 3d10.) I think that pretty clearly demonstrates the faults in this claim.
I will, of course, look up the claimed equivalency numbers later today to get a better understanding of exactly what the designers claimed. But unless you're actually willing to engage with my argument, as opposed to just saying essentially "you're wrong," I don't think there's much more to be said on this front.