Well, I would guess that especially in the US there are more H/J to dressage converts than folks that started out in dressage. The differences may be in where in their riding journey they made the switch.
I got hooked on dressage as an adult re-rider after being a H/J rider for years. I love the partnership with the horse, the feel that I can influence each body part individually (on a good day) and the precision of the sport. That said, it does take some getting used to a different way of riding, sitting differently (be prepared to be really sore in places you were never sore before), and using your aids differently. I still struggle with maintaining consistent contact, although it has improved immensely. If at all possible, get some longe lessons so you can focus on your position and use of the aids.
I don’t do much showing, but two things stand out to me that are different from H/J shows - ride times and score sheets. Shows are run on schedule, and at worst maybe a few minutes off. You are judged against a standard and you receive a score for each movement, and usually a comment, especially if the score was average or low. There are also overall comments at the end and the comments are generally constructive. Although each class is pinned based on relative scores, because you have your score sheet you can “compete” against yourself and your previous scores and assess your progress even if you aren’t in the ribbons.
As far as culture, you can find all kinds of people in dressage from well-heeled folks buying the best of everything to hard-working barn rats struggling with their average horse and minimal lessons and everything in between. There are super competition-focused riders who go for all the year end awards, and riders who never, ever show. The culture at shows is also pretty variable. I am sure there are those that fit the DQ stereotype, but I have found most competitors to be supportive of each other. There are quite a few actual rules in dressage regarding tack and dress and riding the tests, so if you do compete, read the rules. They seem a bit daunting at first, but it really helps to make sure you have your bases covered. Lots of local schooling shows are more relaxed on the rules, particularly with regards to attire and are a great way to transition into dressage shows if you want to do that.
As Rerider54 said, find an instructor and barn that meshes with your goals and outlook. Personally, I can get really anxious and do waaaayyy too many things, so for me a trainer that is more low-key and helps me relax works better than someone who is going to be nagging or yelling constantly.
I have also started my own young horse, something I am not sure I would have contemplated as a H/J rider. He is now 6 and it has been an incredible journey to develop that relationship, and feel the changes in him over time. I took a lesson with a GP trainer recently and she complemented his ability to trust me, and how well we communicate.