DS9 really did develop the characters unlike any other series. A prime example of this is Jadzia. If you were to just take a look at her in any S1 episode and compare it to any S6 episode people might question wtf? She goes from being very Spock-esque to more of a "party girl", but if you watch all the episodes in between you see that at the beginning she is not long Joined, has a lot going on in her head as she tries to find the balance, but then embraces all her previous hosts have to offer and starts living her own life.
On the DVDs the producers said about how she was originally conceived as an old soul in a young body, but found that over time she was the one person always up for an adventure.
All the others developed along the way as well and without too much studio scrutiny and influence (as they were focused on TNG films and then VOY), they could think outside the box and do things differently. I think that DS9 being ignored made it a better series--just imagine how badly it could have gone if it'd gotten the VOY treatment! ::shudders::