I think they had a great group of actors, but yes they were not as well written as they could have been. They were underdeveloped, and I think the reset button at the end of each episode was one of the biggest reasons for this. If everything turns back to the beginning and there are no consequences, how can we expect the characters to grow?
That said, I did enjoy Tuvok, 7 of 9, and the Doctor, of which the latter two were the most well written and sufficiently developed on the show.