Her development this season has been fantastic. Her transformation from the almost perfect girl in Series 7 who saves the Doctor by jumping into his time stream, who convinces him to save Gallifrey, to a more aggressive and bossy girl who is has her own flaws was really rewarding to watch. Her denial of the Doctor's regeneration in Deep Breath and her constant struggles with him have really allowed for some great character moments. As the Series progressed it became clear that she has developed an addiction to her life in the Tardis, and her constant fascination with being the Doctor proved that she no longer has a problem with being a bit ruthless herself. I mean I was totally amazed by the fact that she was going to kill Missy with her own hands, Yes KILL. She has really come a long way. Also her continual lying to Danny just added more depth and relatability to her character.
When watching that last scene in Death in Heaven, I have to say that what struck me the most was her choice of words.
"Doctor, travelling with you made me feel really special. Thank you for that."
Why would she say this? Of all the things she could have said, why this? These lines tell us what she must have been feeling. When she was with the Doctor she was someone really special, someone who was friends with the last of the Time lords, someone who was not bound by the constraints of a mundane and routine life like everybody else, someone important. This is the real tragedy, because the Clara we all know and have come to like really did sacrifice herself for the Doctor when she agreed to say goodbye albeit not in the most conventional sense.
The mid-credits scene however totally nullifies this conclusion. Well I have something to say about that too.