I just feel like it's utterly lacking in the villains department...
Half of the group are more comedic than most previous villains, but they were trying to switch things up, I suppose. I thought that the Zyuranger villains had all around cool designs, so that helped. But Grifforzer and Lammy were pretty decent -- and a threat to the team -- so Bandora's gang wasn't completely incapable. Plus there's Machiko Soga. C'mon.
But, really, I think those designs (monsters of the week included) are pretty cool. And for a more lighthearted show! The ones like Dora Sphinx and Dora Knight, for example, are cooler and look more terrifying than pretty much any Jetman monster, Jetman being Zyu's polar opposite in tone and style.
...and in character development. I could have sworn that the KotW, (Kid((s)) of the Week for those that still haven't seen Zyuranger), got more character development than anybody except Burai.
I definitely think there's a number of episodes where the Zyuranger feel like
they're the guest star of their own show. I think Zyuranger's mistake was rarely giving the individual team members their own focus episodes (maybe I'm just remembering wrong, but most episodes were simply shared by the whole team). That's the way past shows accomplished this -- the adventure the regular hero gets into with the guest character will help shade in some of their own character.
Take a look at episode 44 for how it should have been done. That's really one of Zyuranger's cooler episodes, IMO.