1) Melee Classes: Quite simply Blizzard failed to acknowledge the massive damage gap between classes that can avoid damage entirely (range) and classes that are forced to take damage (melee). Unfortunately, they are just another member of a long list of companies that failed to achieve this balance.
Simple Solution: Almost all melee activated abilities should have a passive defensive included in the core ability. That is, while the wizard is forced to use an entire skill slot for Diamond Skin, the Barbarian (for example) should have diamond skin built into one of its abilities say... Revenge.
2) Elites/Champions: The biggest problem here is not how hard they hit, but the fact that Blizz did not institute a way to avoid the damage like they did with the Act bosses. For example. Why can the "Jailer" automatically encase everyone without missing but Diablo seems to suffer from Down's syndrome when trying to trap people?
Simple Solution: Elites/Champions that are using abilities should have a visual cast-timer (whether it be dark spots on the ground like Diablo’s Cage, or hopping three times before casting) to help players avoid their core abilities.
3) Act Bosses: Seriously, the act bosses should not be easier than the elites/champions. The fact that this is currently the case is inexcusable.
Simple Solution: Act bosses should also gain a randomly generated Elite/Champion ability as well. At which point, they would be "just as hard" as the Elite/Champion packs and should drop loot commensurate with this difficulty change.
None of these suggestions are directed at making the game “more forgiving.” (I agree that failing a mechanic should cost you your life, but as of now there are no “mechanics” to fail you just die). It’s designed to make the game more fun.
Here's hoping to a game that's more fun.