I don't mind having a lot of different humanoid aliens, and I have no problem with 'Forehead alien of the week'. The premises of all the shows except DS9 pretty much demanded different aliens to be present every week, otherwise they wouldn't have been exploration shows.
They should have created a little bit more variety in aliens. Different skin tones, more monstrous looking by human standards. I think the writers felt that the more monstrous looking an alien the less sympathetic that alien would be to the viewers, which kind of goes against the very idea behind the show.
And they could have been more creative with their attractive female actresses, they could have made them look more alien without making them look unsexy. I suppose colored makeup makes their skin look less smooth, but did their range of hair color have to be identical to humans? Give Jadzia blue hair or something. Or maybe Bajorans come in three colors, pink, blue, and green. The pink just happens to be the same tone as caucasian humans, and hey we have this hot caucasian actress, but then there are also blue Bajorans hanging around.