It basically boiled down to the campaigns being for the most part:
a) RPGs few had the patience to go through due to the huge amount of in-game dialogs.
b) Fixed Force scenarios that many people considered to be too hard, too boring or some combination of both.
IIRC, only two campaigns were well received: The Battles of the Forgotten which was pretty much a grind fest B&D, and the Dracula one, that was the least dialog-heavy of the campaigns and had some of the more original gameplay mechanics, like a Risk-like map style in the second scenario.
There were also many complaints about too much eye candy and the horrible path-finding caused by the copious amounts of off-grid objects. And of course, they had no voice acting too, and will never have due to their huge dialogs, from what I understand.
I think the reason for the negative response was because people don't see AoK the way the designers here in AoKH see it. AoKH has a very artsy, story-driven style that suits better RPGs than RTS games. Consequentially, when these campaigns were released (made by members of this community, btw), most people found them inaccessible, as they were looking for some quick fun and not an epic story that shows more technical proficiency than good game design.
The FE team wised up and decided to bridge the gap a little as much as possible with the next expansions' campaigns, making them more ES-like and having a lot more B&D, but with some variation to make them less monotonous. The African Kingdoms and Rise of Rajas campaigns have been well received in general, so I think that, at least in that regard, they succeeded.