You specifically stated 'playing through the game as a cold type sorc, or a melee type sorc. You can't play through the game by switching your skills at lvl60. Nor can you build your character with items useful for such a build. Arcane Orb favours maximum damage per hit. Disintegrate favours maximum damage per second. You can't get both without massive expense.
Honestly everything you've criticised just seems like moaning for the sake of moaning. An expansion will certainly add at least 1 character, probably 2 (2 expansions = 4 characters = 3 per primary stat). They've made it sound like they plan on upping the level cap (hell if I know how they'll make that work) and they'll certainly be adding at least 1 more Act.
I'm not moaning, i'm loving the game. Fact remains that once you hit level 60 with a class you've got no reason to play through it again. You can change your gear at any point if you fancy going a different way with your build but people are much more likely to have gear put away for one type of build and other gear put away for another build. If people decide to level more than one of each class to 60 then they are doing it for the sake of it because there really is no reason too.