I'm with Ironman on this one.
Two supplemental thoughts:
When I had been training for a couple of years, I wanted to tell people what was wrong with their form. After I looked at my motivation, I realized that it was less than because they were going to get hurt and more for my ego. I wanted to prove how much I thought I knew.
I recently commented on two people's form, both regulars at the gym who I know well. The first guy was having back issues and we were discussing his squat. He didn't ask me to, but I watched his form and it was perfect. I told him I had no clue as to why the squat was hurting his back. He reacted positively. The other guy, he frustrates the hell out of me. He basically has a 1/3 bench and is always having problems with his shoulder and is always bitching about it. He is one of those guys who knows everything. One day two weeks ago, he was bitching about his shoulder. I got a lot of crap in my life right now and was in a bad mood. I watched him bench and then lit into him. I told him what I thought his problem was (basically I think is that when he piles on the weight, he goes too far down and since he never benches to his chest he screws up his shoulder). Later on that night, I felt like a total mule. The next time I saw him in the gym I apologized.
Anyway like Ironman says, it usually never ends well.
I know what you mean regarding the knowledge thing, and I thought about it as well, but as I've not been doing this for a very long time, I'd say that I'm rather humble, so that's not my motivation. Some of the cases I might have been thinking of might not even have been 'bad form' as such, but rather guys that are doing bicep curls like mad with way too light weights, while talking to their friends about how they "love the pump" and "feel the burn" and "feel this is going to make them bigger."
A part of me recognizes myself in them when I was younger, and back then I'd have loved having someone to give me a few hints on what might be smart to do for strength gains and so on.
On the fool who knows everything, well, I know that one as well. One of my best mates always talks about wanting to get serious in the gym and so on, then he goes on to show up for the gym like 3 times a month at most, doing whatever he feels like doing, doing ridiculous volume and amounts of exercises for each muscle group and talking about being there for several hours.
Being his friend, I can tell him that his ideas about strength training might not be the .. most optimal ones to have without coming off as a douche, but even in spite of the fact that I've gotten results with how I generally do things, he will at best listen for a while until some other random guy he knows tells him that the best way is to do something entirely different - the type that shows up in jeans and deodorant and wax in his hair, and then sits around doing bicep curls and bench press for the 2 hour workout or something.
Being stubborn and close-minded will not get you very far. But thankfully, those of us who are smart enough to seek information in places where there are people who know more about the subject than we do don't have to worry about that.