She's one of those characters that only really worked in execution by 'tell don't show'. That's one of the great strengths of Who, in that it builds a lot of things through description of importance than a demonstration, this works well with the scope of the show and the fact that the cheesey sci fi style of it will inherently also have trouble 'showing'.
They were suppose to be a great love story, hat's great, but the problem was that they actually then tried to give her a complete story, that included their marriage and her end from his perspective, all while she had a fraction of the development and screen time that a companion was given. Even worse is that she was also something of a Mary Sue and Deus Ex Machina. These are ok qualities to have but not for a reoccurring semi regular character.
She started out strong but the writers are also to blame, especially for the awful 'Marriage' that was tantamount to the Doctor basically acting like he was just doing it to get her off his back.
If they were serious about keeping her around, they should have made her a companion, but also made her younger self be played by a different actress to help give some space between her 'end' when she was introduced, and what she starts out as.
