Okay, got to the character again and this time found a Halberd. That one works. So you're probably right, only "long" polearms can use this ability. However, I found nothing in any of the polearms' descriptions that clearly makes this distinction. Even worse, most of the time even for the ones that don't work claim they're "long", as in the description of the Guisarme:
Quote:
Mounted on a shaft for maximum reach (...)
I don't have a Spear at hand now, but I think its description was even more explicit about length.
So, for future reference, in my experience far reaching attack works with:
but not with:
-Awl-Pike
-Spear
-Trident
-Guisarme
and for any other extra care should be taken before buying a new weapon / selling the old one, despite whatever the descriptions say.