I would describe it as feeling like "quiet". Fluoxetine gave me the ability to quiet bad thoughts. From there, I had more stability to climb out from the pit of despair and anxiety.

The next thing I noticed is that I was able to let small annoyances slide. I used to be triggered by stuff like someone playing music too loudly in the bus. Instead of hyperfixating on that sound and ruminating for the entire bus ride, I could now let it fade in the background and think of something else.

This is the best description of my personal change as well. Medicine won't fix a single one of life's annoyances or solve your problems or help your relationships. What they will do is remove what I would describe as an emotional stickiness.

Like the noisy bus rider you mentioned, that alone is something I can easily deal with now. But back before medication, that frustration would just stick in my brain. And then thoughts of other people doing things from my past would creep in and stick to the initial annoyance. Every minor problem would stick to anything else I could remember, so now instead of a tiny, temporary problem, I had to suddenly deal with every problem I could remember.

Now the thoughts about unrelated things don't creep in, and I can deal with the minor annoyance without everything else.

Other people's comments have mentioned many other effects similar to what I had, but your comment is the only one that mentioned the persistent focus in negative things and the thought creep, or at least that's how I read it.