So, I take it you're for making sure there's no overlap on the Sorcerer, Wizard, or Cleric spell lists, right?
I don't think this has ever been a huge problem. Those classes have always been different enough,
except when you start adding tons of supplements into the game... then it usually happened that there are enough non-core Clerical spells to allow a Cleric to become a Wizard with armor, double hit points, and better attack bonus.
But in core D&D, those 3 classes have always been different enough that some spells in common aren't the problem. Fighter and Rogue are different enough anyway, but if Expertise Dice is given also to Paladin, Ranger, Barbarian etc, there is a real risk that nothing or almost nothing remains unique to the Fighter.
What
really caused the enthusiasm around ED was not the mechanic itself (it's good, but alone it could have not generated such hype): it was the fact that
for once in the history of D&D the
Fighter was both cool and unique.
I understand the desire for diversity, but there's a value in simplicity too - especially in the core. It makes for cleaner balance and easier pick-up-and-play functionality just for starters. One of the major pit-falls D&D can fall prey to is insisting on overloading the core game rules with a lot of layers of complexity in the name of simulation or making every class its own special little snow-flake mechanically.
I totally agree.
So far my favourite 5e playtest package has been the 2nd: every class was still fairly simple, and the Fighter was the only one to have ED, it was his own "schtick"... but to a Fighter (which as a start it's really always the simplest class, with large numbers but no additional tactical features) it still didn't make it particularly complex: spellcasters had spells to learn (strategic choice) and use (tactical choice), rogue had skills and a bunch of extras, fighter had styles for ED.
Now they are adding too much stuff!! Once again all other classes are
more complex than the Fighter. Once again the Fighter doesn't feel so unique.
I'd much rather see diversity in expertise dice consumption and/or generation put into modules. That said, I'd like to see at least some of those module options included in the initial PHB or DMG right at product release.
If we end up having ALL classes with some ED, then I agree... better have ED as an optional module. If everybody has it, it's actually very easy to make it optional.
Especially in an high fantasy system. So imagine you are this fantasy monk, you want to feel awsome and kick this guy on the other side of the room. So you use your special maneuver, fly through the room and kick the guy for X damage and he's projected on the wall where he take some more damage, etc. You feel happy as a super special monk.
...
Expertise dice are basically an at-will ressource but limited by round. I argue that you cannot have cool stuff using a system that is purely at-will and I look at what they give us in this package and that's a pretty big confirmation in that regard.
Well I'm not a fan of superheroes RPG, and this feels a bit like that...
But anyway, you CAN have superpowers in 5e, if you put them into
feats, which you/they are free to design to be at-will, encounter, or daily powers because feats have no strict rules (except the general idea that all feats should be roughly equally worth).