I'm going to be bold and say it. The series is in trouble because of David Tennant.
Now, let me explain. His portrayal was so strong, defined, and energetic, the writers have struggled to write for anyone else. He also stuck around a bit too long. Who struggled after Tom Baker too. Similarly, he was in the role a long time and projected a strong character.
I now feel that Matt Smith's doctor struggled as you could see/hear/feel Tennant's influence on the writing. And they've struggled to break that ever since.
Capaldi was awesome, but they meddled too much with guitars, sunshades and the like. His last year was his Doctor and if that had followed Tennant we may have had the contrast in character we needed.
