The Kazon were terrible, but Voyager had some great aliens too.
The circumstances didn't lend themselves to a lot of recurring races. Since Voyager was apparently going at a steady pace to get home, they couldn't spend as long developing alien races. The fact we got 2 years of the Kazon was a bit implausible, given how much ground Voyager would have covered. It didn't help that they were one of the worst Trek enemies of all time either.
Aside from the Kazon though, I disagree that the races were dull. And the fact they weren't as well developed was just a matter of the fact Voyager was not hanging around in the same general area like the other shows. Many of the more prominent enemies of Voyager only got a few episodes (aside from the Kazon). To compare any of the enemies of Voyager to a race such as the Klingons, whose history has been sorted out over the past 40 years, is a bit unfair.
That said, how many races could compare to the Klingons anyway? They're just badass.
What civilizations were interesting in the Delta Quadrant, besides the Borg?
Here's the problem though, the show ruined any sort of tension early on by dropping the Maquis/Federation tensions and undermining any drama from the crew having to survive in the middle of nowhere when they still had power for the Holodeck and could go on silly adventures whenever they wanted too. So really having underdeveloped alien civilizations who usually provided no tension didn't help with the lack drama.
There were too many aliens who seemed to wanted to take over the ship, for no explained reason. Like aliens from Waking Moments. They are all asleep and entering people's dreams, why do they need Voyager?