>>I've been doing some genealogy work for a while now, and I've now at the point I've got missing cousins and near cousins popping out of the woodwork. Some of my extended family is really excited and asking to see their part of the family tree and all my notes.
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>>My question is - what if you found some not so nice stuff? Like granpa spent some time in jail, or your favorite uncle lied about earning a purple heart in WWII? Would you want to know?
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Here's where my selectivity would come in and it's just my personality. I'd probably tell things about a couple of generations back (where no one knew the person) but I'd hold back knowledge about a favorite grandpa or uncle, just because I liked them and didn't want to tarnish that.
***This does NOT hold true if the crimes were something predatory like child abuse or sexual behaviors, though, because I feel we need to protect everyone from those guys. So, if grandpa went to jail for a hot check, that's one thing, but if it was abuse, I'd tell it. There are some things that seem to follow families, even if not linearly: I had a cousin in his fifties who was abusive to women his whole life. His nephew (not close, didn't live near and never even spent any time with him) is in his twenties and has already spent time in jail for same. They share an abusive female ancestor a generation before that. Gotta be in the blood someplace. So, I think those type things and diagnosed mental illnesses that can be inherited should be told.
>>What about medical information?
Well....I've always heard that people need to know as much as possible about their family medical history. So, I guess I'd tell flat facts like "died of kidney disease" or "died of lung cancer" It might worry them but might also give them some help to avoid problems.
Of course, my real belief is that most people will do what they're going to do, no matter what warnings. (i.e. I should exercise and lose weight due to heart problems in family but I don't.) And, there will always be people who worry.
Another thing is that I would never share ALL my notes with them.
Point them toward some of the sources you used and let them make their own discoveries if they want. You said these people popped out of the woodwork. That indicates you don't really know them.
Give them some information to help them if they're serious about finding out more but don't just hand it all over. I suspect it's mostly an idle curiosity on their part since they've found out you've done the work. That way, they can't be mad because you refused to share any information, either.
And, I'd not make comments on the side because you know whatever you share is going to be e-mailed around the world and it would be the old "Romeo said that your whoever did whatever".......
And, no, I never have an opinion on anything. 
