I recently discussed TAS since I had seen it for the first time, but I had trouble getting through it.
I'm currently watching the entire Star Trek franchise start to finish. I went through TOS and then started on TAS, but despite the cast, I found it challenging to get through at times.
First of all, some of the actors weren't accustomed to voiceover work and it shows. There's a certain vitality from some performers. I love the man, but I have to be honest: Shatner was one of them. Although in the later episodes, you can tell he was starting to figure it out. He's the lead actor, and his energy and screen presence was a HUGE contributing factor to TOS and, yes, Star Trek's success overall, but he was inexperienced going into TAS.
Second, going from TOS straight into TAS was a jarring experience. TOS *always* was crackling with activity. The show was all about energy, activity, action. EVEN if everyone was sitting around, the music, colors, and dialogue kept things moving. Look no further than Captain Kirk. Shatner was a ball of energy on TOS, even if just sitting in his captain's chair...but in TAS, his eyes moved and the rest of him stayed perfectly still. The antithesis of Kirk.
In TAS, the animation was stagnant, had very little activity, and the music was repetitious episode after episode. In other words, the opposite of TOS.
The TAS' main strengths were: A) Bringing back the cast, experienced or not, it helps to have them. B) The writing. There ARE some wonderful stories. Truly worthy of TOS, but the problem was that the animation and music didn't serve them at all times.
Overall, I like TAS. But it does have some flaws, especially when juxtaposed next to TOS.