That's all true, but what I was thinking is that you could take a RQ character into a Stormbringer game and it would be playable - the basic logic of a PC sheet - percential skills broken down across basically the same categories - is common across all those Chaosium games.With Runequest at least it's often a matter of nomenclature. Battle Magic, Spirit Magic, Common Magic and Folk Magic; all the same thing, called by different names in different editions. Bladesharp 2 does the same thing in every case (+10% to weapon skill and +2 damage for the curious). There are systems which have changed, Sorcery is the largest example, but it''s rarely in a particularly fundamental way.
But I don't think they have a very big impact on compatability. An AD&D Assassin ported into a 2nd ed AD&D game will play basically the same as it did in the edition it was ported from. An AD&D thief ported into a 2nd ed AD&D game will play exactly the same, just with less than fully optimised skill percentages.Taking out whole races and classes is a big move.
Likewise half-orcs.
After all, people were bringing new races and classes, or variant races and classes, into their AD&D games all the time, and that didn't change the edition they were playing. Did it?