From more of a business standpoint, I'm sure TV writers wanting to sell scripts probably weren't chomping at the bit to submit for "Star Trek" when the show was renewed against NBC's initial wishes. Seeing that they were going to air it Friday at 10:00pm, let anyone with a brain know, that the show was a lame duck...just hanging on by a thread. Add to that Roddenberry packing his bags and leaving the studio to focus on "future endeavors"...couldn't give the sci-fi or TV writers the warm and fuzzies.
From what I understand, Star Trek had no lack of people submitting scripts. If I am to believe Marc Cushman's "research," more than a few were never used. Trek's problem was being able to film the scripts on time and within the budget. And, honestly, writers just write, and freelance writers write for whatever show they can. Very few writers are so secure they would pass up writing for a halfway decent SF series if they had an idea. Especially a show that wasn't some kid's monster rally. But in the world of TV writers, a job is a job, and a paid credit on a show on its way out is better than no paid credit at all. Actually, I would think that a show with a low rating on its way out would be a good show to submit for. They may be open to more varied material from someone new.
What I just said above also took a negative effect on the actors....Shatner's performance was more broad,
I think this had more to do with the later directors who didn't know how to reel Shatner in, and a producer who thought what he was doing was just fine.
Nimoy as Spock seemed almost depressed and De Kelley just did what he could with the material he was given.
Kelley did fantastic work throughout the series. He was given some of his best material in year 3. The Empath and For the World is Hollow gave him that rare chance to stretch. All Our Yesterdays was also another great showcase for him as he shared most of the episode with Nimoy.
Compare that to Season 1 where you saw almost a different take on Captain Kirk in every episode.
A lot of that was due to their finding their way and the evolution of the character, Kirk was more of the self-doubting "Hornblower" mold at first and gradually moved toward the more confident, heroic Kirk who settled into the second season.