This summary of a YouTube video about him explains it pretty well:
Rick Berman took over showrunning partway through S1 of TNG when Gene Roddenberry's health declined. He only had one real producing credit to his name, and it was for a children's show. Some of the crappy things he did included:
• Being awful to women actors, corroborated by Denise Crosby, Marina Sirtis, Terry Farrell, Jeri Ryan. He gave them zero respect during contract negotiations, increasingly wanted to sexualize their roles (catsuits, "decontamination" scenes, etc), and straight up fired Terry Farrell from DS9 after she just asked for an episode reduction for S7.
• Screwing over Wil Wheaton by forcing him to turn down an opportunity to star in a big movie for no reason other than control/power
• Denying Garrett Wang the opportunity to direct an episode of Voyager, even after a bunch of main Trek actors have directed episodes before
• Attached his own name to the DS9 episode that introduced the Maquis, despite not having anything to do with the episode, because he was afraid Ira Behr would want royalties for VOY (since the Maquis were integral to VOY). Ira had no intention of this, it was Berman being paranoid and having a lack of trust.
• Consciously and continuously steering away from any references to gay people, despite numerous good script pitches and actor pitches. Notably, had completely genuine, organic opportunities to turn Garak and Malcolm Reed gay with full support of the actors, but instead completely steered the other way and put them in situations to "confirm" their straightness.
• A general attitude toward safeness and dialing back any bold actions. DS9 writers basically had to go behind his back to do certain things, and he hated the show as a result
• Also hated the idea of serialized storytelling because he believed all the money was in syndication, where single episodes needed to stand on their own.
• Told the human actors on VOY to intentionally be boring so that the alien actors would seem more realistic. Numerous scenes had to be reshot because the humans seemed too emotional.
• Increasingly stuck his nose in the writing credits for VOY and ENT (even though he wasn't a writer!) as the franchise began to spiral downward. Once he gave showrunning control over to Manny Coto for S4 of ENT, the show became good, although by then it was too late.
Jadzia was easily one of the best parts of DS9. I had no idea her departure was because of a request like this. That's pretty stupid.
Take a fan favorite character and fire her because she asked for a bit of a break? That's completely idiotic. I'm not going to say that Ezri was bad or anything; but the fact is that Jadzia was great and replacing her was entirely the decision of one small man who couldn't give enough respect to his co-workers to respect their wishes all because they were women.
I can only imagine what could have happened, and how great things could have been if this one fool was ejected sooner.
Nicole de Boer did a great job as Ezri and the writers did what they could with her character, but she always felt shoehorned into the show. The fact that her character intentionally didn't fit in did not help at all. I guess they wanted her to be the opposite of Jadzia? And I really didn't like the idea of the Dax symbiont put into the closest available Trill because it was an emergency plot device.
IMO, they just wanted to explain why dax was staying with the show. I'm sure they had future episodes in mind where they needed sisko to call some woman "old man" for some reason. Rather than writing that out, they just made to some crap to make it "make sense" why dax didn't leave ds9.
Apparently it's easier to rewrite Jadzia into Ezri and make her rather incompetent in the process. IMO, the biggest loss was that Jadzia had a certain sass to her that was entirely lost with Ezri. I don't blame Nicole at all, as far as I'm concerned, she did an excellent job, it's just how the character was written. Poorly. Dax as a character became timid and submissive, compared to the commanding presence that Terry had with the character, it was rather different. IMO, it would have made more sense to have dax, the symbiote, depart the show with Terry/Jadzia, and just bring in Ezri as a young officer instead.... Just write out the whole simbyote entirely.... But no, they needed a reason for Sisko to give her the time of day. Not because he respects her as a person, or a citizen of the federation, or an officer of star fleet, but because she used to be a dude that was friends with him.
Idiotic.
When I think back on all the ds9 stuff with this context in mind, it paints a grim picture of what Berman was all about. Though you can see examples of his unique personality in other shows, I think DS9 has some of the most blatant and obvious portrayals of what he wanted, reflecting who he was as a person. It's just, not a pretty picture.
I'm glad that Ezri Dax got better fleshed out in the books. Nicole de Beor was a great addition to the show and they did a lot with her in the short amount of time left. But the reason she was an addition to the show was just so stupid.
She certainly had big issues with Berman and I sure would have gotten fed up and left, but she's obviously a consummate professional and put up with it for the sake of her craft.
“The problems with my leaving were with Rick Berman. In my opinion, he’s just very misogynistic. He’d comment on your bra size not being voluptuous. His secretary had a 36C or something like that, and he would say something about, ‘Well, you’re just, like, flat. Look at Christine over there. She has the perfect breasts right there,’” revealed Farrell.
“And then I had to go into his office. Michael Piller didn’t care about those things, so he wasn’t there when you were having all of these crazy fittings with Rick Berman criticizing your hair or how big your breasts were or weren’t. That stuff was so intense, especially the first couple of years,” she added.
Jadzia Dax’s death had the total lack of dignity that I associate with Rick Berman. She didn’t just die suddenly and rather pointlessly. She twitched for a bit while the camera lingered. I really can’t stand this man.
TNG is actually my favorite series, but I'll admit it doesn't really get good until S2. By S4, it's fantastic. I'd highly recommend giving it another shot