Merry Christmas.
To be honest, I suppose it's safe there's going to be a lot of changes implemented to skills, stats and spells so it's hard to figure out how the balance will be in the end.
Currently, all I have to do to win fights is spam my AoEs of Doom so I can't really judge the tactical potential of this game. Maybe charge times and some shared cooldown will be the solution to it all, I don't know.
Aramintia, why I think infinite skill slots work in other games and may not in D:OS is simply because D:OS doesn't use classes but a freeform system.
While I enjoy the ability to build my heroes as much as anyone else, I've also played in the past too many games where the freeform would lead to have "tankmages with healing factor" trivialising the content. Believe it or not, limiting the amount of spells/abilities available at once is more than an "artificial quickbar limitation".
I suppose we can say it's more a core gameplay mechanic similar to how, in card games (either recents one like Munchkin, Bang!, Magic or traditionnal ones, I don't know if you play any of these : p), you have a limitation on how many cards you can keep in your hand.
