The dislike 5 because the atmosphere, and that Heroes "feel" is just gone. Remember how, back in 3, visiting buildings gave you that little bitty text about what's happening there, and then told you what you got from it - that's gone; now it's just a floating number showing which resources you got. Same for the artifacts. In my opinion, this lessens the game's value considerably - now it's just another strategy, and not //Heroes//.
On the other hand, you can see that HoMM 4 was made by 3DO, because they DO have all those fun little things. I suspect HoMM 4 was mostly disliked because it was such a drastic change. Also, the initial release was plagued by bugs, crashes etc. I, personally, quite enjoyed it for what it is (ie. not a "regular" Heroes game).
In the end, HoMM 5 gives less attention those little important details, and the only innovation it brings is the initiative thingie. Everything else is just plain recycled from old games (I don't consider allowing heroes to attack a major change, since it's virtually the same as casting a spell, which was always present).