DuracellEnergizer said:

The overuse of human and rubber forehead aliens.

This says it all ^.

When the show got so complacent that it would introduce us to new Alien races week after week with ever so slightly different nose jobs, was when the problems started.

Then when it got to DS9 they thought it was exceptable to have the main Alien race (The Bajorans) that the entire show revolved around, have four nose ridges and an earing?! When you had Babylon 5 airing at the same time who had wierd and freaky aliens (with peacock hair, ornate bones growing out of their heads, full reptillian facial appliances or no physical presence at all) then your franchise starts to look tired.