I really enjoyed this map.
My suggestions:
1) I couldnt see much of ANYTHING in the 4th act. PLEASE get rid of the danged cloudy crap. I fumbled around forever, especially in the diablo sanctuary. Ugh. Adding options for removing the cloud cover would really enhance the map so you arent just fighting red blobs that ALT key tells you exist.
There is an unintented problem that caused it around 9:30 pm and 2:30 am. It is because after writing the code to use variable fog, z-start, z-end, density, and color, I ended up changing default camera parameters to show more area. Most of these can be fixed by reworking the table of z-end values, so that light will reach the ground on that time interval (mostly visible in act4). I hope to work on this on an update. For now you can use the -cam1 and -cam2 options, or just use the mouse wheel to get the camera closer.
2) The casters need some kind of a basic attack. Melee is a bit ridiculous. The casters in D2 spam their spells, not just use them sparingly. Necromancer could use a magic attack which simulates the Teeth spell. Or make Bone Spirit a 5/10/15/20/25 mana spell that adds a large amount of damage to an attack. They should never melee.
This is because the spells were left mainly in a proof of concept state. I wrote initial code for more spells, so that different builds could be made. And I wanted to allow melee and ranged builds. Just that I gave up after some working on animations... I wrote a set of python scripts that allow editing Warcraft models in Blender, but never finished it in a truly usable state.
It shouldn't be hard to give them a ranged attack tough, just change staff and wand items to work like a bow, and possibly give it a different attack. The teeth idea could be done using Barrage as base ability (but probably it should be made as a hero passive ability and not an item ability).
3) More balance needed. The Amazon is a paragon of awesomeness. She has a summonable tank with incredible toughness. An uber lightning spell that is BETTER than the spellcaster's abilities. A so-so cold attack, and a passive that more than DOUBLES her effective damage with the first point! Subsequent points have minimal effect though. I was able to win with her.
Yes. The Amazon is the strongest character, but that is due to the Valkyrie, but Critical Strike needs a nerf. Skeletons are stronger then the Valk, but they need corpses, and in this map, they aren't as common in the places they are mostly needed because of monsters that don't leave a body. In an updated, I will probably give skeletons hero resistance, or most likely something like the Barbarian Natural Resistance spell, that is a buffed Elune's Grace ability.
Necromancer skellies are too weak and get owned by area effect spells. Maybe have some of them become skellie MAGES at higher levels to reduce clutter and increase firepower.
Sorceress just needs some better offense. Freezing sphere would be awesome if you could reproduce it. She needs some slow effects badly as she has NO tank. She definitely needs to not melee...
Frozen Orb probably could be done using Vorpal Blades as base ability, to give the piercing effect. Her slow effect is Shiver Armor (based on Frost Armor), but that means she needs to get hit by melee attacks to actually cause the slow effect, and some bosses (Duriel, Izual and Diablo) have a ranged attack (tagged as ranged, but of range 100, i.e. melee range, so that they can freeze opponents; I could not find a way to cause freezing using normal melee attacks).
Pally and Barb seem underpowered to me, although I didnt give them a fair trial.
3) Add more stuff to the F9, like how to save your game by talking to Cain!!! I accidently figured this out.
How bout a few Hoadric cube recipes? Maybe the potion upgrade ones? And what about the perfect gems? Same as real D2? (i.e. useless)
All classes can finish the map. Probably the weakest character is the Paladin, but still he got a good nuke with 2700 damage, just needs some hit and run on bosses (bosses don't regenerate life), because he is vulnerable to spells, but very strong agains't physical damage. The barbarian has no nukes, but he can tank Diablo for almost one minute as long as you put like 2 points in vitality per level.
A perfect gem, plus any life and potion mana can be transmuted into a rejuvenation potion.
I have been away from Warcraft for some time, but I played the map to ensure it is still working with the current patch, and finished it a few times.
In an update, I should also reduce mana cost for Paladin and Barbarian spells.