I used Kismet for years and fought with its limitations on a daily basis. By far the biggest limitation of Kismet was that every graph was tied to a level. If you wanted to do any general purpose programming, such as mechanics shared across every level, you had to duplicate your graph for every single one of them. It was a very tedious and error-prone process.

Blueprint is leaps and bounds ahead of Kismet. It's built for general purpose programming out of the box. You can write your own classes, components, enums, and more. You can expose parameters to the Details panel so that designers and artists can easily tweak classes on a per-instance level. You can still write a graph tied to a level a la Kismet — this is called a Level Blueprint — but I find it's rarely necessary, if at all.

I remember trying UnrealScript because of Kismet's limitations. It was certainly more powerful and expressive, but shutting down the editor every time I wanted to compile was a real drag. These days I've gotten used to the fact that Blueprints are compiled on the fly by clicking a button in the editor, with no interruption to iteration. I can't imagine going back.