Star Trek improved in inverse correlation with Gene Roddenberry's direct influence over it.
He was a big-picture man, not unlike George Lucas or Terry Nation. Inventive, but not brilliant when it comes to continually grinding out new material in the same space. Much of the 'meh' around Star Wars 1-3 can be attributed to Lucas having become so powerful, there was no longer anybody to tell him if he'd had a crap idea. And if you watch Blakes' Seven you can see it visibly running out of steam after about half a dozen episodes and suddenly starting to look a lot like mid-Baker-era Doctor Who without the time travel.