DS9 is my second favorite Trek(close second to TOS), so there is little I actually hate about this show.
But with 7 seasons there are always some elements or episodes you don't really like. In television no show is perfect, even your favorite show.
But I will list a few, and I'm sure some of these have already been mentioned here (I'm new to the forum and am at work and don't have time to read all 70 or 80 posts right now).
1. Ezri Dax. Unlike many of the posts I have had time to read, I actually like Jadzia. I agree that Farrell isn't the best actress in the world, but I think she was adequate for the role. But Ezri annoyed me greatly. If Farrell wanted out, fine, they can't help that. But they could have given her a better exit, and then just drop the character completely. Ezri was unnecessary, and was so out of place on DS9. Whe would have fit better on TNG with Troi to hold her hand.
2. The finale. I understand the importance of heroic sacrifice in drama, but I just didn't feel like Sisko's apparent death(I know he wasn't really dead, but that was almost worse for those he left behind) was necessary.
3. Keiko. I just don't see what O'Brien sees in her. She creeps me out everytime I see her. As a character she has absolutely no warmth or interest.
And like any long-running show there are several eps that I just didn't particularly think were good.
But on balance, especially compared with TNG, VOY and ENT, it was about as good a Trek as we could hope to get under the circumstances.
As I've said before, it was the most TOS-like Trek in my opinion.