Heroes of Might and Magic 4

Objectively speaking, HoMM4 was not a great game. Its AI was as sorely lacking as that of every other installment in the series. It shared their common flaw of a massive build-up of troops actually countering the attrition from which it would logically suffer. It was unfortunately geared toward campaign more than freestyle gameplay.
But that all amounts only to saying HoMM4 was an HoMM game with HoMM flaws. It should be viewed as what it was: a last-ditch attempt to revolutionize an aging series by a developer already spiraling the drain after failures with other projects. Its various features must be addressed in context and occasionally offer some important insights into game design.

Heroes as units
The biggest shock to most fans was seeing their beloved heroes thrown into the battlefield, open to harm, and though a few years later Warcraft 3 would make this a standard expectation for fantasy RPG heroes, HoMM4's variant was not re-integrated thoughtfully enough into gameplay. Their main role remained to cast nukes, heals and debuffs just like the invincible, standoffish heroes of previous games, so placing them in the field served no purpose, especially since their spells had no minimum ranges and very limited line-of-sight issues.

Hero skills
Instead of developing one core hero class, each hero could mix-and-match skills to become a prestige class. So Life+Death magic = Dark Priest, Nature+Death magic = Demonologist, etc. Unfortunately this removed heroes' personalities, which was counterproductive to Heroes 4's emphasis on a slightly more mature RPG setting. Any demonologist was the same demonologist.
Also since hero survivability depended predominantly on one single "Combat" skill class, this combined with their new overall vulnerability made it too much of a must-have.
In fact, balance was almost completely thrown out the window here. Given the HoMM series' emphasis on avoiding attrition, having a combat hero absorbing hits on the front line or using summoning skills for the same reason or any other such measures proved stupidly overpowered.

Faction types
Abandoning the traditional knight/sorceress/wizard/etc. split of previous HoMMs, #4 shamelessly cribbed the magic color system from Magic: The Gathering, with black/white/red/blue/green and nonmagical alignments. This in itself was good as it allowed for faction alignments for neutral creatures and rooted these distinctions more intuitively into the setting, but unfortunately was not taken far enough. Some of the magic spells for instance were quite unimaginatively pasted between factions, especially when it came to all-purpose magic missiles. Still, it was a big step up from HoMM3... or 5 for that matter.

Mobility
Just as heroes could for the first time move around without creatures, creatures could travel without heroes, in any size stack. While open to abuse and too reliant on micromanagement for lack of automation options, this helped flesh out the world even more, removing the single-minded focus on the "Heroes" of might and magic. But then is it really a Heroes game? Regardless, I thoroughly enjoyed it and have only recently realized that one thing which drew me to Elemental from the start was this very same feature. However, since HoMM still placed no limit on the number of creatures in one stack, splitting them off into their own armies was pretty pointless, so this promising feature came to nothing in the end.

Fewer tiers
Most HoMM games have depended on seven incrementally better creature tiers, each slightly better. #4 instead reduced this to four tiers. This made the power difference between tiers much more satisfying, but removed the strategic choice of skipping tiers while upgrading a town. However, it left an interesting niche for neutral creatures as intermediates: elementals were somewhere between 2 and 3, zombies between 1 and 2, and so forth.
However again, the vast difference in power made some creature special abilities interestingly if aggravatingly overpowered when facing lower tiers, like vampires' lifestealing or gryphons' endless retaliations. Others like minotaurs, which had a chance to block any physical attack regardless of the amount of damage, or medusae which sheared units off the top of a stack regardless of tier, had the opposite effect.
Also quite importantly, this eliminated creature redundancy. No more pointless, perfunctory upgrading of creatures to bigger badder versions of themselves, and no more endless varieties of green dragons, red dragons, gold dragons, pink dragons, purple chipmunk polka-dot dragons...
Variety is not variety without distinction.

Building choice
Here was an excellent replay value improvement. Instead of allowing players to fully upgrade any town, tiers 2,3 and 4 of the creature generators in each town presented two mutually exclusive choices. Can't build golems if you choose magi, that sort of thing. So you could specialize in demons one game and undead the next, or mix-and-mach as you think best.
This was also about the only feature thoroughly tested and balanced so that no choice felt extraneous and all had their purposes, which could not be said of the hero skill and spell choices.

Finally, there were some features which were unambiguously good or bad.

The new combat map, replacing the traditional hex grid, was a monumental failure. While other flaws could be overlooked, the awkward, irrationally twisting paths which creatures walked in combat were a constant frustration.
However, it did offer the important improvement of allowing full line-of-sight blocking of ranged attacks.

Aesthetics were hit-or-miss. Decent sound, good writing, bad graphics. Neither realistically proportioned nor the adorable cartoonish storybook tropes of past games, creature models dipped into grotesque uncanny valley caricature. The music though was pleasingly over-the-top and always apt to its surroundings. In fact, one of the saving graces of this entire game series through the decades has been retaining a competent composer.

The writing deserves special mention because of Heroes of Might and Magic's traditional role straddling the TBS and RPG genres. Though the storytelling side in most iterations has been so painfully childish as to not be worth mentioning (no really, I immediately gave up on HoMM5's campaign mode after the first time they shouted "Griffin eternal!") HoMM4 made a big point of wiping the slate clean and introducing a new, slightly more Grimm faerytale world. It's not really up to true RPG standards, but there is some pain and desperation, fear and survival in the stories of Emilia, Tawni or especially Gauldoth.
Just as importantly, the writing team made a decisive effort to immerse you in that world through the pop-up texts attached to mundane actions like finding a new shield or buying a skill at an interactable map location.

Overall, I would definitely recommend this to anyone interested or experienced in TBS games or the RPG/TBS mix. However, I don't think I'd recommend it to anyone just casually taking a first look into the genre or the HoMM series. Heroes 3 or 5 were both more polished and... representative.