I thought it steadily declined after season two.
I actually agree. It seemed like the show tried to deny its roots...which actually put me off by the time season 3 hit....
No Andorians, changing the look of Vulcans and Romulans, no Orions, Tellerites to mix with the new aliens that were created. Not too mention, changing the uniforms, and taking away the 'fun' and adventure aspect of Trek.
With that said, there were some few good episodes in later seasons.