Xeriph:
Mechanics
The system was written for DnD, but as i recall the only things that mattered really was if you were arcane/divine/not caster, and a few skill ranks like diplomacy and such.
There was a bit more to it than that. Fighters and rogues had different domain types they excelled at, for instance, With Rangers splitting the difference between them, much as Paladins could go Fighter or Clerical domain. Mind you, they might not be as effective as a dedicated Fighter, Cleric, or Rogue.
I think Bards split Fighter, Rogue and Mage. It's been awhile. Still, class had a fairly large impact on options, at least if you wanted to benefit form a domain under your control.
Converting to a later edition of D&D is probably fairly simple, but to another game system, especially one without classes or with different classes could take some work. Depending on the game it could require enough of a rewrite it might not even be Birthright, anymore.
And let's not forget the option to Adventure as your realm action for a season, and, possibly, gain divine power (I forget what it was called) by killing endowed opponents (and possibly their foreign domains). Or just finding some treasure to pay the bills.
I'd be willing to give a modernized D&D game a try, but I have my doubts about it being converted to a different game system and still be recognizable as Birthright.