Goseiger is not perfect, but I think it tends to be under-rated. Looking back on it, the thing that stands out as a negative is the overal plot. The idea they had for the main villan is not a bad concept, but the first and last villan group plot arcs are too short. The first ended too early, at the point where one (or at least I) was getting used to/interested in the Warstars. At the time my reaction was - "it can't be over yet!" along with confusion. And the last set of villians was around for such a short time, they seem to have been put there simply to make the point of the main villian showing up again. The set that was around for the majority of the show was nothing exceptional.
And I was dissapointed with the super forms and weapons. The super forms do not look good and seem to have been just an imitation of past ideas, done just to do a super form.
Now Yudai Chiba, Goseiger's red, I was happy with. The opening of the show focuses on him helping the boy and acting like a guardian angel. Kudos. Yudai is not a stereotypical red in many ways, but I don't see the loss of a "screaming red" as a negative. The character seems to have been intended as the opposite of a screaming red, thoughtly, kind and even sensitive - and Yudai Chiba brings plenty of energy to his role when needed. I think Yudai did quite well with his role, and the rest of the main cast, along with Gosei knight, at worst, did decently. I liked the character focus episodes and though the relationship develoment with Gosei knight was good.
So if i was to sum it up, I would say the characters/actors made the show for me, and the plot and super forms/weapons were the bigest disapointments. Oh, and they went overboard on the cards. I know the show exists to sell toys, but the cards were not imagintive and seemed more like a sales pitch the switches or keys or medals.