I didn't mean to imply the writing is poor... far from it. But, the plots are far less engaging than the characters, dialog, and scenery (for me).
When I finish with series 4, I am going to give "Upstairs, Downstairs" a try. I have heard some fans describe Downton Abbey as "Upstairs, Downstairs for morons".
I prefer the original 1970's Upstairs, Downstairs more than Downton Abbey, perhaps because Downton seems like a knockoff of U,D to me. I would definitely give U,D a try if you like Downton. They did a new U,D that brought back one of the original characters a couple of years ago, but it wasn't very good so it only lasted 2 seasons.