I actually want to hear you elaborate on this. I don't really hear people trash on DS9 too much, despite the fact that it's definitely not without it's flaws.
I’m more of a TNG guy myself (grew up with that show — I think there are literally baby pictures of me with TNG on the tv in the background) and I haven’t gotten into DS9 yet, despite trying. The following points come from a biased point of view.
I think we don’t hear much criticism of DS9 these days because Star Trek fans are in general older and look back at 90s Trek fondly. Hell, we’re seeing people here and in the main Trek thread lionize Voyager and Enterprise (remember how the internet destroyed those shows before nuTrek? Pepperidge Farms remembers). It’s a little bit like what’s going on with the Star Wars prequel revisionism (as much as people hate admitting it, the fandoms have a lot in common).
The biggest criticism of DS9 I got from friends attempting to watch it and much older family members around when it originally aired was that it was more soap opera than Star Trek (I think my mom jokingly called it “General Hospital in space”). For every bit of scifi world building, or intergalactic political intrigue (the “good parts” of DS9), there were 5 soap opera scenes of interpersonal relationship drama. If you know and care about the characters, it’s tolerable and even fun, but for the audience that was used to TNG, it was whiplash.
That leads to the second major criticism of DS9 — it wasn’t TNG. While it’s thrown around as an insult, the “anomaly of the week” format is what people LOVED about Star Trek. DS9, taking place on a stationary space station was not that and was harder for the TNG fans to get into. Later seasons, when they brought Worf on and had the USS Defiant, looked like a cheap attempt to draw TNG fans into DS9 with mixed success. The arguments between more casual TNG viewers and the established DS9 fans leads to the next point.
The third criticism I can think of, but it’s more mild and indirect, is that the DS9 fandom, at least in previous decades, was insistent that DS9 was the best Trek ever and shut down old farts who want their TOS/TNG anomaly of the week. I remember seeing the “you just don’t get it” excuse used by DS9 fans frequently and I think that annoyed older TNG fans and more casual viewers.
Are these valid? Maybe, but they seem pretty silly and petty these days. I think without nuTrek, old nerds had less stuff to argue about and made mountains out of mole hills.
My favorite way to trigger a haughty DS9 fan is to say it’s a rip-off of Babylon 5. The seething you get is quite amusing.
