(Sorry OP, I don't mean to hijack your thread.)
Well, my letter first says that I'm interested in the school and why, and then has an update. Since I interviewed on their first interview day, months and months ago, there is actually quite a bit to say: what I've been doing in my job, extra shadowing I've done, volunteer work with my synagogue, working on an election campaign, and activity with another political organization. So to describe all of that takes space, especially the part about my job, because I'm getting basically constant clinical exposure so I want to discuss that.
I do wonder how much I should say about the political stuff. I want to mention that I'm campaigning for a Senate candidate, and I think that's enough (without mentioning party affiliation), but as far as my other stuff, it's with a specific healthcare group, and just by mentioning the name it's obvious what their political platform is.
I'm not sure a bulleted letter would really work. I would stick to business letter format.