Interesting question... There are two sides to this, I think - I find that rule systems inform a particular style of play, so any rule that engenders an enjoyable style of play is going to be liked, but just answering that aspect of the question will end up in this thread becoming a discussion around what style of play people like. As this is a Dragon Warriors forum, I see another side to the question - whether there are any rules\mechanics implemented in other systems that could be ported to Dragon Warriors to reinforce a low-fantasy pseudo Dark Age style of gaming that makes DW unique.
There are loads of great rules in other games, but when I play Dragon Warriors I don't want to romp around Faerun or Barsaive as an untouchable hero with nothing to fear from the repercussions of my actions from the common folk. I want the social constraints to impose themselves on even a high-ranked character - a sorcerer of any rank has to hide himself away far from the peasant mob to perform his blasphemous experiments, the high-rank knight still unquestioningly follows the orders of the low-ranked baron, and the high-ranked mystic must still be careful using his gifts in public in case they are perceived as witchcraft rather than a blessing from God.
Pendragon has mechanics to enforce knightly virtues and the romantic Arthurian style, but whilst there are some great rules that would port nicely into DW (a lot of estate management stuff and, not surprisingly, stuff for adding depth to DW knights, for example), a lot of it would only work for Pendragon. Similarly, the Conan RPG is probably the best comprehensive handbook for barbarians out there, but only a small sliver of that content is suitable for porting into Dragon Warriors
There are, however, some game concepts (rather than rules...) from other games that I think would work in Dragon Warriors:
- I like that Dark Heresy has the concept of professions, that branch as the character increases in level (similar to the way a Sorcerer branches into demonology in DW)
- I like how the Lone Wolf d20 game made no bones about the professions being equivalent in power - Kai Lords and Shianti Wizards clearly had superior abilities on paper, but ALL of the professions had a unique role-playing niche in the game and reasons to play them other than how many abilities they had.
- I like how in Earthdawn, magic items grow in power as you learn about them and invest your personal power into them (Vallandar's swords in King Under the Forest are a little like this).
- I like how the Basic RolePlaying skills system means it can take a LONG time to get good at anything.
- The feats system of D&D is an interesting approach to Skills of the Mighty (some feats could even be ported across as skills of the mighty for some DW professions).
- And so on...