Personally I can't stand the kagillion skills you have in WOW. 8 is alot less, but I would hope that at least half of those are passive. Having to manage more than 4 activable skills is a mess. Sure it may distinguish the ranks by skill, but don't forget there may be activable items too!
Personally, I'd like to see every Demi with 2 core activable skills. Then maybe 1 or 2 more situation/ultimate activables, and then manage the items as best you can. Fill up the rest of the slots with passive skills or auto casts (blah). Anymore than that will cause you to pull your hair out and the lesser skills will just go unused.
Boo to 40+ demi's. Dota has a large community that is constantly churning out new hero ideas. Sometimes, there's a really good proposal so they add it. They can't cut any of the existing Demi's because the group that plays that hero (and every hero has fans [except maybe Keeper of the light]) would complain. Dota has evolved into many characters. Earlier versions had fewer heroes to choose from.
To illustrate, think back on Street Fighter 2. There were only 8 selectable characters with 4 bosses (unlockable in some versions). The game was fun, you could play through a brief story mode with each character. Every character was built to be able to take on just about any other character (some exploits, but thats beyond the point). It didn't take too long to get a feel for what each character could do, how to play, and how to counter.
Now think on Marvel vs Capcom. I think there were about 70 characters. Sure, it may be fun for variety, but how long did it take to learn all of their skills? Maybe you just pick a handful and learn them, but you would still need to know what the other characters could do so you can counter them properly. IMO, Streetfighter 2 was the better game.
I'd say give us whatever Demi's you have ready for the beta, be that 4 or 12. Demi like dota, will have up to 5v5 on the field, so at least 10 unique Demi's will be needed for release. To give everyone some options, I see around 20 for release. After that, either patch or add-ons could slowly introduce more Demi's. I think they will lose flavor after 40 though.
my 2 cents