Since I watched the first two episodes of S3 season finale, I went to YouTube to get a general idea as to what happened in the last episode besides Doctor and the world surviving. You are absolutely right about Martha, she is very well-adjusted, self-confident, smart, etc. Basically, she knows when to get out of the dodge in all situations, including a romantic attachment. Her last scene with Doctor and her decision are completely level-headed, like most of us would make in a similar situation in real life. A part of me agrees with you that a character being so normal doesn't make for a good ride, but a part of me was glad to see someone like us traveling with Doctor for a little while. I personally cannot imagine myself in place of Rose, but totally can in place of Martha.