Game esthetics isn't the only issue, that make HOMM2 better single player game than the others. Of course, the animation of "armageddon" spell or fire breathing black dragon are great and still look loveley, compared to HOMM3 ugly tactic battle map. But the gamplay is also better in many points:
- the game itself is harder and more challenging, while the wandering creatures are more difficult to kill, resources are scarce, and your towns give you much less tax revenue;
- the towns themselves are more balanced, giving much more variety at the same time; i.e. castle and barbarian towns don't have any flyers but have some great foot units, whereas warlock castle has three flyers, but only one weak shooter
- the potentialy weaker towns are faster and cheaper to bulid and the same goes for cheaper units
- the best last level creatures are very expensive and hard to get (you need great amount of sulfur or gems, to build upgraded titans or dragon dwellings) which makes the iportance of the other units last longer in the further part of the game, whereas in homm3 especially archangels give you even greater adventage than black dragons in homm2 (becuse of their resurection spell) but their dwelling is quite easy to build, making other castle units obsolete in the middle part of the game;
- not all dwellings can be upgraded which is a good thing too - if all buildings can be upgraded, than there's no bonus for towns with weaker, but upgradable creatures;
- there is no town in homm2 that you can consider weak, becuase all of them have their own strenghts and weaknesses, wheras in homm3 some towns are really overally weak, and the othere evidently strong (while human players are able to use the adventages of weak but faster to build town, A.I. seems to have problems with that in homm3, but still is formidable oponent in homm2);
- magic type skills in homm3 are fun, but make some spells way too easy to get, like mass haste and mass slow (which were third and fourth level spells in homm2) town portal (which was practicly unavailable in homm2 - I only got it once in my life), ressurect true or armageddon;
- becuase of that, getting as much as possible magic type skills for your heroes is a priority, but all four of them occupies half of the skill slots, leaving you with only four slots for other skills (considering, that also wisdom is a "must", and intelligence is one of the best skill, it leaves you with two slots for non-magic skills);
- some skills in homm3 are really useless, like mysticism compared to intelligence (in homm2 mysticism was quite useful, because spells were cheaper to cast, there were less wells, no mana regenerating artifacts and less knowledge increasing artifacts; the usefullness of ballista skill (with so many strong flyers in homm3 in every town) artillery (ballista is really weak), first aid (first aid tent is really easy to destroy) or learning is also questionable;
- in homm2 there was also a good balance between footers, flyers and shooters (like in paper-rock-scissors game) - shooters were strong against the footers, but weak against the flyers, wheras flyers were strong against the shooters, but weak against the footers (in homm3 there is no such balance);
I guess, these are the main things, that came to my mind comparing homm2 to homm3. Of course there are some things, that homm3 done better than homm2, i.e.: more space for artifacts and the artifacts system itself. But then again, some artifacts in homm3 are way too strong and make game way too easy.