The more I think about it now, the more I think that the rules should be extremely light and you just let things happen very organically. You let people run groups the way they want (as long as the global rules are maintained) - if people don't like it, they fork the group
So what.
For commercial stuff, I really think now that the "no commercial advertising" rule is all that is necessary. If companies want to create fan groups, go for it. This site is about people and if 1000 people sign up for a fan group of company XYZ, who has the right to say they can't or there should be a rule against? Don't sign up yourself
I don't think we need to moderate group approval, rather let the community report them if inappropriate (otherwise you really have to moderate events and goodness knows what else). The only exception to this could be that the project has final say over some group names that they might want to reverse for official working group purposes.
For events, if you really want to post your birthday ... does it really matter? Do we really need a rule about that?
Regarding duplication of Joomla resources, that was an interesting post from Claire. I initially thought, yeah good point, but then I thought does it really matter if someone creates a group that duplicates the showcase for example. Either it will be well attended or not. And if it is well attended, the problem is not the group, the problem is why haven't we given the person doing it a job
Regarding spam, the only thing that's starting to irk me are the email invites to join a group. I don't consider the activity logs "spam" by definition, but there is certainly a signal-to-noise ratio and ultimately that comes down to personal preference as to what is acceptable or not. No amount of rules are going to help satisfy everyone. The solution is to continue to improve the technology so that you can keep the signal strength high (that is, the stuff you are *really* interested in) and the noise low (you can't cancel it out completely).
Anyway, I think the rules are as tight as they need to be for now. Fine tuning might be needed down the track obviously, but overall the groove seems to be quite good.