When you submit to KDE you should see a template. Follow the template, or at least include all the content expected by the template. We will have lot's of proposals and it really helps us to learn a bit about you, your timeline and a lot about your proposal. Assume that the person reading the proposal isn't familiar with the projects you are working on. It's a good assumption: KDE is very big, the people reading your proposal probably aren't familiar. :) So explain why your project idea is needed, even if it came from the project idea page (you've already been studying that page, right?). Get into the specifics of how its going to work. Successful KDE GSoC submissions typically go through quite a bit of revision, so its a good sign if you receive feedback.
But most of all, if you are a student with a free summer, give it a try. :)