Please add Counterspelling to the Player Tools, so it can be set to the inventory as a quick slot and then clicked on an enemy caster. Currently you have to go through a number of awkward radial menus to counterspell.
I believe - and correct me if I am wrong - that the reason counterspell was disabled for quickslotting to begin with was that quick, immediate access to the ability is buggy and severely abuseable. First of all, that would require a very close examination.
Keep in mind, as well, that if we could do this, it would be strongly dependent on extension from external software, which I am not 100% sure is immediately available off the top of my head. It isn't a feature of vanilla NWN. Maybe one of my colleagues can comment on what existing extensions are out there already.
If not, it would require a disproportionate amount of work to develop/launch a plugin for this one little feature, which we might not even want anyway.
Johannes is correct.
There was a bug a while back where you could que any number of spells and then counterspell, and then still be able to cast these spells after the fact, but this was fixed in 1.67. Now counterspelling is stuck being a clunky not often used ability. Alas, that it can't be done though.