For me the biggest advancement in DF to come is when the world becomes active while your playing Fortress or adventure mode i.e Towns grow / armies move / wars start and end / bandits roam.
Not sure how far off that is but from Toady's notes it appears that its not going to show until around 0.5, the scariest part is I can picture myself at age 40 ( Im 23 now ) explaining to my wife and kids why DF 1.0 is the coolest thing ever, Also maybe my Toady crayon art will be super valuable in 100 yrs time, like a picasso.
...this is literally the goal of the version Toady is developing right now.
Praise the Sock!
The Sock ?
Also, you shouldn't treat groups as merely sets because you would have a tendency for them to include only the information contained in the elements which compose them. Groups ARE objects, with sets of members (and yes, some fuzzy logic will most likely be required for things such as "loyalty" or "likeness of objectives").
The reason why is, there is some information about the group that is not its members nor its name. For example, objectives, ethics. Even if you could derive the new objectives or ethics of a group from statistics, from a programmation point of view it makes more sense to have a group use the same interface as an individual, in the case of a friend that ask you not to do something versus your religion that asks you to do it.
Plus groups like individuals have things they overall like or dislike, objectives, relations to other entities and can be part of other groups. Even if the inner working of a group's mind are partly derived from the minds of its members, there is benefit in regarding them as objects with sets.
(Plus, but it's quite impractical for DF purposes, some would argue individuals are themselves sets of simpler entities. cf. Minsky's society of mind)