One of the reasons Trek has always appealed to me is because of it's optimistic vision of the future.
Stay away from DS9, then. You're in for a surprise.
Too late, watched that before I started Voyager, haha.
I loved DS9. No, it wasn't optimistic in the way that TNG was. It revealed the darker side of humanity. I prefer TNG's vision, but because of just how well DS9 was written, directed, acted, etc. I still thought it was an incredible show. The moral dilemmas were fascinating because of how grey they were. To this day I still don't know how I would have acted had I been in Sisko's position in In the Pale Moonlight, to give an example.
I think that dark and pessimistic can be fine for Trek as long as it's presented in the right way. DS9 accomplished this brilliantly.