I feel like a lot of people are mistaking gmail's opinionated UI design for a design failure. Changing the subject isn't difficult because the designers failed at making it easy, it's difficult because the designers succeeded at making it difficult. They really, really don't want you to change the subject because consistent subject lines are essential to gmail's conversation threading functionality, so they've made it as difficult as possible.

similarly, the hiding of all the formatting options: they have all that space available, and they've quite deliberately chosen to hide things anyways. there's no way they just did it to make the compose window prettier, they want you to stop using so much formatting in your emails. Read the interview[1] with gmail's designer, they are trying to use a simpler UI to encourage faster and smaller responses and lower friction conversations. By and large, people are using email as a messaging tool, not to sit down and spend half an hour composing a letter, and they are adapting gmail to cater to this behaviour. you might disagree with their goals, but don't mistake your disagreement with their direction for a failure to advance their direction.

[1]http://www.fastcodesign.com/1672250/how-a-tiny-new-compose-w...