Upgrades were really just bloat. They tried to make them something beneficial to non-spellcasting castles when they added them, or alternatively give lousy units to spellcasting guys but with the option to get them better with upgrades. In practice it didn't work very well. They were a clunky and unnecessary thing.
As for some ways in which H4 "sucked" as listed here
"completely busted unit balance" - Most other Heroes games didn't have enough unit variety to have balance issues and still managed to have plenty of balance issues. There's just nothing going for Inferno in Heroes 3, for example.
"the huge drop in unit variety" - not really true, the combat system (and spellcasting) made units way more distinct than they ever were. Heroes IV has startling ammount of variety with fewer troops per castle, and being able to edit who is in your army and take some guys out and put some guys in on the fly actually leads to more varied combat than any other Heroes game ever had.
"the retarded participation of heroes" - meh, it wasn't nearly as bad as folks make it out to be.
"the idiotic castle sieges" - that they were. Although they were pretty retarded all the way from Heroes 1. These were just retarded in different ways.
"the dumbed down spellbooks" -
No, there was never a deeper, more interesting and meaningful spell system in a Heroes game before or after. You can bash Heroes 4 on many fronts, but this isn't one of those.
"the much less readable overland map" - yep, that it was, and IMO ugly, but being able to split individual units off combined with caravarns was so good all could be forgotten.
"the loose munsters running around the maps" - yeah, that was a bad idea. You could turn that off, mind you.
"the clunky as hell battle maps" - "MAH HEXES!"
"no hero specialisations" - "MAH ENGDAME SPELLS AT LVL1" (And there sure were specialisations)
"no level 1 hero differentiations" - What are you even talking about? A Lvl 1 Druid was a completely different thing than a lvl 1 Priest, like, so different that you were essentially playing a different game.
"and the list goes on" - Yes, except it only continues along the way you're going if you take even harder drugs.
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Also, don't even bother trying to interacti with the vaguely polish sounding Z guy, he's just an (unintentional?) troll. There's noting but deep fanboyism in there.