Some classes, like paladin or bard, got their gameplay mainly based on reaction, so I'm really curious to see how Larian will implement it (and maybe that's why they didn't released those classes yet)
For example, about the paladin, Divine Smite can be cast AFTER you made your attack, and so only cast it on a critical roll, or choose a low level spell slot on a normal hit and a big one on a critical. It's why that ability is really powerfull and without that, paladin will be way weaker.
Right now, you only have the choice to cast Divine Smite before your attack is made, and the spell slot is used only on a hit, but it's not enough.

Wait and see...