That's funny. I just wrote the exact opposite. I think the show started getting better when Henry Blake and Trapper left. Really? Wow. I prefer the early years, because it was so zany. I think the tone of the series changed after Henry/Trapper left. There were some good episodes in the later years (most of them dealing with Winchester in some way).
I prefer the Henry Blake-isms over the Sherman Potter-isms. ;)
Kevin