Ahh, I guess my friends must have been a little more on the midcore side or above going from WotLK to Cata because they loved Cata heroics. I did as well.
Nonetheless, I don't think its difficulty came down to a sweeping HP and/or "deadliness factor" buff across all mobs so much as a failure to rein in very specific mob effects when scaling up and how those changes forced awareness and decisive counterpoint. This left a much more punctuated difficulty that then felt quite manageable to very skilled players while seeming both unintuitive and overwhelming to less skilled players. Where a skilled player might look at the damage intake and immediately takes up a kiting pattern, that degree of adaptibility just wasn't available to many an average player. And given some of the weirdness that went on there, with certain mobs scaling far more rampantly than others from Normal to Heroic by nature of their skills, I always that as a failure to polish quite enough, since difficulty as a result of damage intake or output required isn't... linearly perceived(?) (if there could even be such a thing), especially once the floor has already been raised significantly.
The last point about Mythic+, though, is one I hope XIV will learn from. I like Mythic+ as an efficiently designed reiterative system, but it is far from perfect (or all that interesting fleshed out in terms of rewards), and in this case I would the failing you describe, for instance, comes from only ever trying to suit its needs for general grinding and gear progression simultaneously.
Let me exemplify. I see no reason why I shouldn't be able to take an underperforming player to content in which they would otherwise drown without (1) risk of failure or (2) making the difficulty excessively low for myself as well if there isn't anything about the content that would specifically require, somehow, that my lowest DPS be nearish the highest in party (e.g. by split-party mechanics or whatever or tight DPS checks). Though, if it's really just about grinding, and it isn't that much less efficient to just step down a level in difficulty, it's almost a bit irrelevant; the only true solution at that point is to have dynamic mechanics also tied into the difficulty level and to target those mechanics at specific players based on their individual difficulty levels selected, so to speak.
That said, it makes no sense to then treat gear progression as if it were mere grinding. That certainly doesn't happen in raids, so why should someone who can't beat the fights in X time at Y difficulty (the equivalent of clearing any boss Y in a raid, we might say) be rewarded with that same loot? Removing the Mythic+ timer would be the equivalent of removing every enrage from raids and making the excess time purely a point of vanity. I can't take Dyslexic Joe to the Linguomancer raid fight. It may suck, but that's just the fact of it. Why would I expect any different, then, from something that provides the same quality of loot, even if based on a dungeon model?
I'd thought a lot about a reiterative system very specific to XIV that gives more control over the elements of difficulty added by dividing up the different types of difficulty that may be added to a dungeon by Element, collecting and deploying TT Cards for optional bonus fights a la Necromimicon, and using player-based difficulty levels (an idea partly borrowed from 1.x, and partly from multiplayer Rogue-likes) in the form of Blessings/Challenges, with slightly different workings between premade and matched parties -- the general idea being to give a more interesting and player-directed reward structure via Relic Armor (aka Regalia) and the gathering of Element as an analog reward structure punctuated by progressing tiers therein for interesting materia effects, preferably while allowing greater skill gaps within any given party without creating conflict for that party's workings.