So I have installed WBC3 really recently (and may uninstall soon as I do not play games much anymore), but when I did play I recall being a real fan of WBC 1 & 2.
So for the newbie question:
I have found links to several mods that are available and seem to have some support as of not so long ago for WBC 3, but very little info as to what the Mods change and which are compatible, and which people actually play. So:
1.)
a) Which is the best mod? (I don't care about story, campaign etc, but I mean spell/race balance wise)
b) Which mod is played the most?
c) Does anyone play this mod online?
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2.)
It really has been awhile since I played anything, but there were several obvious defects and flaws that struck me while playing of late.
a) Terrible hot-keys - is there a way to rebind/assign hot keys to skills/attitudes etc. ?
b) Really bad level system - I might be wrong but I was under the impression that in WBC2 there were various different bonuses you could buy depending on your race/class aside from the regular attrib points. Here as far as I see it there are many skills that are plain worse than others (and accordingly will never be bought since there is no scale: 1pt from 0->1, 1 pt from 29->30). Is there any mod that makes something decent of the leveling system?
3.) Non-newbie questions and notes (from what i've seen so far).
There have been very few decent hero builds that I have actually found.
<Combat/Warrior-type heroes. >
For example it is hard to understand what reason there is to play a 'warrior-type' hero rather than an Assassin. Just the raw stats for Assassin are nearly as good (dexterity is better scaling than strength - another defect) in the long run, he is more competent for map/control conversion and better in combat.
The only possible argument for a 'warrior type hero' is something like a Paladin, that can support and lifeward. But the problem is that lifeward is no cheap spell, accordingly a great number of attrib/skill pts need to be diverted to int/magic in order to have decent casting, which in effect makes you a much crappier fighter.
<Mage/Caster type heroes>
Most magic seems to... Well to be kinda weak... I mean compared to just having a high level tough guy. Now I know Mages are supposed to get better with player skill, but many of the spells just don't cut it...
The purely offensive schools seem to be too weak to justify playing a low str, low dex, slow run, low hp, slow conversion hero. The only thing that really stands out is Necromancy (great synergy with undead) and Healing (support for tough units, like Knights). Healers no longer get Arcane, as of the official 1.03 patch, which has definitely made them weaker.

And schools like Chaos Magic, kinda just make you wonder, what the designers were thinking when they took the time to code something so obviously stupid...
<Utility/Spec Heroes>
I haven't played the merchant/engineering types, so I can't say much about those. A thief hero certainly brings a lot to a fight, (fast travel low conversion times), personally I've found that having poison magic works well on a Thief but I assume some other magic schools might work too, it doesn't make sense making him into a fighter, since than you would just be playing an Ssrathi or Barbarian assassin to begin with. If I am somehow very much/completely wrong on these impressions feel free to comment. Looking forward to a response.