Personally I think it went wrong before it went into pre-production. The premise of "let's get back home" had been done to death well before then, and it forced the series' writers to try to come up with new, episodic content after 10 years of TOS+TNG had done so. Also, it basically competed with DS9, which hurt the franchise as a whole.
Maybe if they had waited 2-3 more years it would have been better for Trek.
Just my opinion, of course. I tried to watch the show for the first two seasons but I gave up after the 3rd season opener.