If someone asks me what was the main difference I noticed between the 2 conferences, I would have to say: attitude. They were both small conferences, but even though most of the people in the symposium knew each other, there is always this "frenemy" vibe I get in scientific conferences, whereas at the ACS everyone was so inviting and welcoming...or at least that's how they made me feel. I even talk to some well known characters in the field, and while I did feel a bit self-conscious (it is me after all) their tone was never patronizing and I not once felt like anyone was looking down on me, even if I was one of the new ones.
Another big thing I noticed was that during the talks and workshops, most of the questions in the ACS one felt constructive. It's not that the questions in other type of congress are destructive per se, but understandably most of the time, there are questions intended to discover the flaws in a present hypothesis. While this can make the research stronger, there is always a way to be critique of someone's work.
Finally but probably the most important one is the communication itself. The language used. While it is to be expected that a scientific conference has much more specific terms during a talk, the thing is not everyone attending is already an expert. People jump into acronyms or terms that might be very obvious to them and their lab, but there are so many of them (acronyms) that you can see how slowly a big chunk of the audience just disconnected. If you don't know what NTD means (Neglected Tropical Diseases in this context) chances are your brain will wonder off trying to figure out what it is, and by the time you ask/figure it out, the talk has advanced and you just missed the whole relevance or the punch line. You could argue that the people in the ACS managed to pass their message along way better, because they precisely work in communication. But if the whole point of going to a conference is sharing your information, scientists have no choice but to step up their communication game.
In all guys, I feel like I am making the right choice in this career path. You know when you find a group of friends and you sort of feel at home? Because everyone understands you in a very weird way? That's how I felt.