A thought: unlike reactive blocks, reactive counterspells don't really change the power of a spellcaster compared to other PCs, because any 'caster with channeling/evocation would have the ability. This leads to, in my view, more interesting dynamics when you have several opposing spellcasters in play. Wizard ends up countering wizard, and then the physical characters go at it. It's possible this method reduces, to a degree, the common issue of shorter conflicts favoring spellcasters over physical types. Instead of (or in addition to) the Paranet Papers' introduction of reactive blocks as a standard aspect of evocation, why not add reactive counterspells?
I probably wouldn't allow a Warden's sword or other enchanted items to be used reactively this way, only actually-cast spells.
Thoughts?