Well, i can't say this for sure but with a little insight i have. Most negative feedback has come from the heroes being all the same. And tank model isn't good enough for hard tanks. Too little str and base hp regen same as others. Also missing of 0.00 caster is very bad thing i've heard.
Also i would add for the diversity of the game not the"making all same" concept. This just my suggestion: Instead of creating classes and making all models basically same hero i would and i think many "long timers" who have hard time to adapt to the new system would agree more to a change like you made with some heroes earlier (buffing) like pit lord and tauren chieftain clinkz, meepo, not in a radical way like these were buffed maybe a little too much but with the base point of normal dota heroes and then with "common sense" you could take a look at each hero individually and see what they miss compared to others and how could you "balance" the situation a little. So every hero would have his pros and cons.
for eg. We could take now (1) centaur/tidehunter, (2) invoker/leshrac (3) naix/alechemist for good examples.
(1) Centaur has good stats, movement and statsgains, little bad base dmg and cast points but suits very nicely for a tanky build. (mostly picked hero for tanks)
Then is tidehunter, you probably dont see him like ever in games, unless somebody want to pick it for the lols (like me) and own with it. But anyway, tidehunter has fairly bad stats, bad gains, also bad cast time his only perk is his movement speed. Then you could think how to get people to pick him but also keep the game diverse and heroes separate. Well you could make him a little more updated to anyway like giving him a fairly better mana gain and cast time alongside with his normal movement speed. So people could use it as caster for example or anyway you want to change them.
(2) Invoker/Leshrac
Invoker is fairly bad in everyway except he has awesome cast time which makes him a valuable choice for only caster. His movement speed is just very terrible but lets keep it that way. Buffing him with a good gain of int and keeping his slowiness. Then there is leshrac with good movement speed and intellisgence. Also he is very seldom seen ingames because he has TERRIBLE cast time, but lets keep it that way so he suffers from bad cast time but then he could be used in some other builds. Lets upgrade his stat gains (str/agi and drop his BaT. Now he is suited for other type of builds like (impetus) and makes a valuable choice for it.
(3) Naix is allaround hero and one of the most picked heroes of older lods (6.74) He has a little bit of everything. Not very good statgains and bad armor. But he is very fast and quick to cast with a good attack animation/cast animation. Lets keep him as he is. Comparing alchemist then. Alchemist is probably never seen ingame (except his ultimate skill) he has bad stats and gains and he is slow and dont have armor or damage, he could be implemented with better movement speed and like very good agility gain for str hero.
So my conclusion would be that instead of classifying all models i think they should be upgraded and the game could be made more diverse. Give some hero good stats but he is a slow caster or has bad attack animation. Somebody very bad statgain but make him Insanely fast in everything with attacks cast times, movement speed. One could be slow but has very high mana, some could be allaround "good" in everything but not awesome. Other could have very high range but thats about it for him. So every hero would be unique and separate from other but put all together they would suit for their own role.
I know this would be a lot of work to do and probably lots of tweaks would be needed after release but already after the "upgration" of heroes on earlier versions (pit lord meepo etc...)was mainly positive experience i've seen. And people started to pick them but also picked the older ones. They had probably too radical buffs and made them the new most picked but if every hero could be tweaked to its unique purpose with their pros and cons i think it would be succesfull.
Feedback appreciated from both administration and dota communities is apreciated.