This is better than a stat system since the powers occupy item properties. And one could immediately re-spec his character stats or roll another hero to deliver the needed requirement for an item, but one can’t find immediately a new item with more tuned stats.
Ultimately the system could be extended with all things you wrote like vitality, wisdom etc buffing one’s character stats.
For example, let’s say a player stacked 10 powers on items for a total of 150 points. Then in our system if the player weighted 50% to procs and 50% to wisdom, half of these 150 points would serve for proc buffs and half for wisdom buffs. Then when the player finds and equips a new item without particular power losing 20 Demonic points, the weight would still be 50/50 to procs/wisdom, but these would have with 10 less points each.
Yeah, Diablo 4 looks really promising! They just have to make a longer BETA so these kind of stuff is polished before release.